Fixes transaction cookie accumulation that caused 413 errors when users repeatedly navigate to protected routes. Adds configurable cleanup with three new AuthClient options for different application needs.

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

🔍 RCA

Each startInteractiveLogin() call created new __txn_* cookies but only deleted the matching callback cookie, leaving abandoned cookies from incomplete flows to accumulate until hitting HTTP header limits.

📋 Changes

Adds configurable transaction cookie management to prevent accumulation while supporting multi-tab scenarios. New AuthClient constructor options:

• enableParallelTransactions: boolean (default: true) - Controls single vs multi-transaction mode
• Added maxAge property to TransactionCookieOptions to control txn cookie maxage (3600s by default)

Usage example for single-transaction mode:

new AuthClient({
  // ...required options
  transactionCookieOptions: {
    // ...options
    maxAge: 60 // 1 minute
  }
  enableParallelTransactions: false,
});

Changed src/server/auth-client.ts: Added configuration options and cleanup in startInteractiveLogin()
Added tests/v4-infinitely-stacking-cookies.test.ts: Comprehensive test coverage for all options

📎 References

Fixes: #1917
Fixes: #2209

🎯 Testing

Automated:
Added comprehensive test suite in tests/v4-infinitely-stacking-cookies.test.ts covering all configuration options, cleanup scenarios, and edge cases. Fixed CI test failure by providing explicit fetch mock and routes configuration instead of accessing private AuthClient properties.

Manual:

1. Navigate to protected route multiple times and verify ≤3 transaction cookies remain
2. Test multi-tab login scenarios work correctly with enableParallelTransactions: true
3. Test single-transaction mode with enableParallelTransactions: false
4. Run pnpm test:unit to verify all tests pass