fix: update transaction cookie deletion to clear all cookies on succe…

[rajyan]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

This PR fixes the transaction cookie accumulation issue by calling deleteAll in handleCallback upon successful authentication.

Problem:
In v4, transaction cookies (__txn_*) accumulate over time as users navigate the app while unauthenticated. This eventually causes 431 errors due to request header size limits.

Solution:

• Changed handleCallback to call transactionStore.deleteAll() instead of transactionStore.delete(state) after successful authentication
• This ensures all accumulated transaction cookies are cleaned up when a user successfully logs in
• Complements the existing deleteAll call in handleLogout

Changed files:

• src/server/auth-client.ts: Replace delete(state) with deleteAll() in handleCallback
• src/server/redundant-txn-cookie-deletion.test.ts: Update test to verify all transaction cookies are deleted on success

📎 References

• Fixes v4: Infinitely stacking cookies #1917 - Transaction cookies accumulate infinitely causing 413 errors
• Related to PR Fix stacking transaction cookies #2077 which added deleteAll to handleLogout

🎯 Testing

Unit tests:

• Updated redundant-txn-cookie-deletion.test.ts to verify that all transaction cookies are deleted after successful callback
• All existing tests pass

Manual testing (verified locally):

1. Started the app and navigated around while unauthenticated (created multiple __txn_* cookies)
2. Logged in successfully
3. Verified all __txn_* cookies were cleared from the browser