test: adds multiaccount v4 mocks (#25094)

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## **Description**
Adds mock for accounts api in multiaccount v4
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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adds coverage for the multi-account balances endpoint used in E2E
tests.
> 
> - New GET mock for `v4/multiaccount/balances` in
`tests/api-mocking/mock-responses/defaults/accounts.ts`
> - Returns sample native balances for `eip155:1` (ETH) and `eip155:137`
(MATIC) with `unprocessedNetworks: ['eip155:1329']`
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
183c81c. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: christopherferreira9

tests/api-mocking/mock-responses/defaults/accounts.ts

@@ -1314,6 +1314,45 @@ export const DEFAULT_ACCOUNTS_MOCK: MockEventsObject = {
         unprocessedNetworks: [],
       },
     },
+    {
+      urlEndpoint:
+        /^https:\/\/accounts\.api\.cx\.metamask\.io\/v4\/multiaccount\/balances\?accountAddresses=.*$/,
+      responseCode: 200,
+      response: {
+        balances: [
+          {
+            count: 2,
+            balances: [
+              {
+                object: 'token',
+                address: '0x0000000000000000000000000000000000000000',
+                symbol: 'ETH',
+                name: 'Ether',
+                type: 'native',
+                decimals: 18,
+                chainId: 1,
+                balance: '0.000000000000000000',
+                accountAddress:
+                  'eip155:1:0x76cf1cdd1fcc252442b50d6e97207228aa4aefc3',
+              },
+              {
+                object: 'token',
+                address: '0x0000000000000000000000000000000000000000',
+                symbol: 'MATIC',
+                name: 'MATIC',
+                type: 'native',
+                decimals: 18,
+                chainId: 137,
+                balance: '0.000000000000000000',
+                accountAddress:
+                  'eip155:137:0x76cf1cdd1fcc252442b50d6e97207228aa4aefc3',
+              },
+            ],
+            unprocessedNetworks: ['eip155:1329'],
+          },
+        ],
+      },
+    },
     {
       urlEndpoint:
         /^https:\/\/accounts\.api\.cx\.metamask\.io\/v\d+\/accounts\/\/.*$/,