tests(clerk_flutter): improving ClerkSignInPanel and ClerkSignUpPanel tests

[MarkOSullivan94]

Resolves #428

Why

The ClerkSignInPanel and ClerkSignUpPanel tests under test/integration/ were a solid starting point for exercising the sign-in and sign-up flows, but they run with flutter_test against mocked services rather than on a device, so they are widget tests by Flutter's definition — and with Patrol now covering true end-to-end flows (#410, #412), the folder name no longer reflected what they do.

While moving them, I noticed the tests could only verify that ClerkSignInPanel and ClerkSignUpPanel themselves were present in the tree: both panels build an empty widget when no Environment is available, and the test mocks only supplied a Client, so no form fields or buttons were rendered beneath them. Supplying a mocked environment lets the tests assert on the actual UI for each scenario.

What

Test support

• Added createTestEnvironment(), building an Environment from required/optional user attributes (with matching verification strategies) and identification strategies. The two panels have different rendering gates: ClerkSignUpPanel is driven by env.user.attributes, while ClerkSignInPanel requires a non-empty env.config.identificationStrategies.
• createTestAuthState() now accepts an environment, served by TestHttpService from the mocked /environment endpoint.

Test files

• test/integration/* → test/widgets/clerk_sign_in_panel_test.dart and test/widgets/clerk_sign_up_panel_test.dart, one file per widget under test.
• Each test now supplies an environment matching its scenario and asserts on what actually renders (form fields, strategy buttons) rather than just the panel's presence.
• Consolidated overlapping password-factor scenarios in the ClerkSignInPanel tests into one test each for: password, email code, password + email code, phone code, and second factor (TOTP + backup code).
• Aligned group/test naming with the existing unit tests: groups named for the widget under test, test names as behaviour statements (renders X when Y).

Test plan

• flutter test in packages/clerk_flutter: 779 passed, 7 skipped (count down from 781 due to consolidating duplicate scenarios).