fix: remove dead error branch in handlePostSocialLogin (#27578)

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## **Description**
Removes dead code in handlePostSocialLogin where the else branch for
result.type !== 'success' was empty with only a placeholder comment.
This branch is unreachable because OAuthService.handleOAuthLogin()
throws on failure, and the error is caught by the try/catch in
onPressContinueWithSocialLogin which routes to handleLoginError →
handleOAuthLoginError → captureException (Sentry).

Replaced the empty else block with an early return guard and a comment
documenting the error handling flow, improving code clarity and
preventing silent failures if the flow is ever refactored
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CHANGELOG entry: null

## **Related issues**

Fixes: https://consensyssoftware.atlassian.net/browse/TO-594

## **Manual testing steps**

```gherkin
Feature: Social login error handling

  Scenario: user completes social login successfully
    Given the user is on the Onboarding screen

    When user taps Google or Apple login and completes authentication
    Then the user is navigated to the appropriate next screen (ChoosePassword, AccountAlreadyExists, etc.)

  Scenario: user encounters an error during social login
    Given the user is on the Onboarding screen

    When user taps Google or Apple login and an error occurs
    Then the error is caught by handleLoginError and reported to Sentry
    And the user sees an error toast and remains on the Onboarding screen

```

## **Screenshots/Recordings**

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before and after of your change. -->

### **Before**

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### **After**


https://github.com/user-attachments/assets/0073d035-a502-4fdd-abdb-afb9bbe5cc47


<!-- [screenshots/recordings] -->

## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [x] I've included tests if applicable
- [x] I've documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I've applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk refactor in onboarding social login flow: it removes an
unreachable branch and adds an explicit guard, without changing the
success-path navigation or state updates.
> 
> **Overview**
> `handlePostSocialLogin` now *explicitly guards* against `result.type
!== 'success'` with an early return and documents that OAuth failures
are handled upstream via `onPressContinueWithSocialLogin`’s try/catch
and `handleOAuthLoginError`/Sentry.
> 
> The success-path logic (account type dispatch, metrics event, and
navigation for create/import + existing/new users across iOS/Android) is
preserved, but is no longer nested under an `if (result.type ===
'success')` block.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
6c98ea9. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: grvgoel81

app/components/Views/Onboarding/index.test.tsx

@@ -1310,6 +1310,56 @@ describe('Onboarding', () => {
       );
     });
 
+    it('does not navigate when OAuth login result type is not success', async () => {
+      mockCreateLoginHandler.mockReturnValue('mockGoogleHandler');
+      mockOAuthService.handleOAuthLogin.mockResolvedValue({
+        type: 'error',
+        existingUser: false,
+      });
+
+      const { getByTestId } = renderScreen(
+        Onboarding,
+        { name: 'Onboarding' },
+        {
+          state: mockInitialState,
+        },
+      );
+
+      const createWalletButton = getByTestId(
+        OnboardingSelectorIDs.NEW_WALLET_BUTTON,
+      );
+      await act(async () => {
+        fireEvent.press(createWalletButton);
+      });
+
+      const navCall = mockNavigate.mock.calls.find(
+        (call) =>
+          call[0] === Routes.MODAL.ROOT_MODAL_FLOW &&
+          call[1]?.screen === Routes.SHEET.ONBOARDING_SHEET,
+      );
+
+      const googleOAuthFunction = navCall[1].params.onPressContinueWithGoogle;
+
+      mockNavigate.mockClear();
+
+      await act(async () => {
+        await googleOAuthFunction(true);
+      });
+
+      expect(mockNavigate).not.toHaveBeenCalledWith(
+        'ChoosePassword',
+        expect.anything(),
+      );
+      expect(mockNavigate).not.toHaveBeenCalledWith(
+        Routes.ONBOARDING.SOCIAL_LOGIN_SUCCESS_NEW_USER,
+        expect.anything(),
+      );
+      expect(mockNavigate).not.toHaveBeenCalledWith(
+        'AccountAlreadyExists',
+        expect.anything(),
+      );
+    });
+
     it('attempts browser fallback when no credential is available in Android', async () => {
       Platform.OS = 'android';
       const noCredentialError = new OAuthError(

app/components/Views/Onboarding/index.tsx

@@ -412,83 +412,85 @@ const Onboarding = () => {
         socialLoginTraceCtx.current = undefined;
       }
 
-      if (result.type === 'success') {
-        const accountType = getSocialAccountType(provider, result.existingUser);
-        dispatch(setAccountType(accountType));
+      // Error case (result.type !== 'success') is not handled here because
+      // OAuthService.handleOAuthLogin() throws on failure, and the error is
+      // caught by the try/catch in onPressContinueWithSocialLogin, which calls
+      // handleLoginError → handleOAuthLoginError → captureException (Sentry).
+      if (result.type !== 'success') {
+        return;
+      }
 
-        track(MetaMetricsEvents.SOCIAL_LOGIN_COMPLETED, {
-          account_type: accountType,
-        });
-        if (createWallet) {
-          if (result.existingUser) {
-            navigation.navigate('AccountAlreadyExists', {
-              accountName: result.accountName,
-              oauthLoginSuccess: true,
-              onboardingTraceCtx: onboardingTraceCtx.current,
-              provider,
-            });
-          } else {
-            trace({
-              name: TraceName.OnboardingNewSocialCreateWallet,
-              op: TraceOperation.OnboardingUserJourney,
-              tags: getTraceTags(store.getState()),
-              parentContext: onboardingTraceCtx.current,
-            });
+      const accountType = getSocialAccountType(provider, result.existingUser);
+      dispatch(setAccountType(accountType));
 
-            if (isIOS) {
-              // Navigate to SocialLoginSuccess screen first, then  ChoosePassword
-              navigation.navigate(
-                Routes.ONBOARDING.SOCIAL_LOGIN_SUCCESS_NEW_USER,
-                {
-                  accountName: result.accountName,
-                  oauthLoginSuccess: true,
-                  onboardingTraceCtx: onboardingTraceCtx.current,
-                  provider,
-                },
-              );
-            } else {
-              // Direct navigation to ChoosePassword for Android
-              navigation.navigate('ChoosePassword', {
-                [PREVIOUS_SCREEN]: ONBOARDING,
+      track(MetaMetricsEvents.SOCIAL_LOGIN_COMPLETED, {
+        account_type: accountType,
+      });
+      if (createWallet) {
+        if (result.existingUser) {
+          navigation.navigate('AccountAlreadyExists', {
+            accountName: result.accountName,
+            oauthLoginSuccess: true,
+            onboardingTraceCtx: onboardingTraceCtx.current,
+            provider,
+          });
+        } else {
+          trace({
+            name: TraceName.OnboardingNewSocialCreateWallet,
+            op: TraceOperation.OnboardingUserJourney,
+            tags: getTraceTags(store.getState()),
+            parentContext: onboardingTraceCtx.current,
+          });
+
+          if (isIOS) {
+            // Navigate to SocialLoginSuccess screen first, then  ChoosePassword
+            navigation.navigate(
+              Routes.ONBOARDING.SOCIAL_LOGIN_SUCCESS_NEW_USER,
+              {
+                accountName: result.accountName,
                 oauthLoginSuccess: true,
                 onboardingTraceCtx: onboardingTraceCtx.current,
                 provider,
-              });
-            }
-          }
-        } else if (!createWallet) {
-          if (result.existingUser) {
-            trace({
-              name: TraceName.OnboardingExistingSocialLogin,
-              op: TraceOperation.OnboardingUserJourney,
-              tags: getTraceTags(store.getState()),
-              parentContext: onboardingTraceCtx.current,
-            });
-            isIOS
-              ? navigation.navigate(
-                  Routes.ONBOARDING.SOCIAL_LOGIN_SUCCESS_EXISTING_USER,
-                  {
-                    [PREVIOUS_SCREEN]: ONBOARDING,
-                    oauthLoginSuccess: true,
-                    onboardingTraceCtx: onboardingTraceCtx.current,
-                  },
-                )
-              : navigation.navigate('Rehydrate', {
-                  [PREVIOUS_SCREEN]: ONBOARDING,
-                  oauthLoginSuccess: true,
-                  onboardingTraceCtx: onboardingTraceCtx.current,
-                });
+              },
+            );
           } else {
-            navigation.navigate('AccountNotFound', {
-              accountName: result.accountName,
+            // Direct navigation to ChoosePassword for Android
+            navigation.navigate('ChoosePassword', {
+              [PREVIOUS_SCREEN]: ONBOARDING,
               oauthLoginSuccess: true,
               onboardingTraceCtx: onboardingTraceCtx.current,
               provider,
             });
           }
         }
+      } else if (result.existingUser) {
+        trace({
+          name: TraceName.OnboardingExistingSocialLogin,
+          op: TraceOperation.OnboardingUserJourney,
+          tags: getTraceTags(store.getState()),
+          parentContext: onboardingTraceCtx.current,
+        });
+        isIOS
+          ? navigation.navigate(
+              Routes.ONBOARDING.SOCIAL_LOGIN_SUCCESS_EXISTING_USER,
+              {
+                [PREVIOUS_SCREEN]: ONBOARDING,
+                oauthLoginSuccess: true,
+                onboardingTraceCtx: onboardingTraceCtx.current,
+              },
+            )
+          : navigation.navigate('Rehydrate', {
+              [PREVIOUS_SCREEN]: ONBOARDING,
+              oauthLoginSuccess: true,
+              onboardingTraceCtx: onboardingTraceCtx.current,
+            });
       } else {
-        // handle error: show error message in the UI
+        navigation.navigate('AccountNotFound', {
+          accountName: result.accountName,
+          oauthLoginSuccess: true,
+          onboardingTraceCtx: onboardingTraceCtx.current,
+          provider,
+        });
       }
     },
     [navigation, track, dispatch],