feat: Add flag to control parallel transactions

[tusharpandey13]

This PR adds an Auth0Client flag enableParallelTransactions?: boolean that controls behaviour of SDK for parallel transactions.

This PR also adds some logic to ensure that transaction cookies are cleared in the event of a callback error.

• 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

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

Usage example for single-transaction mode:

new AuthClient({
  // ...required options
  enableParallelTransactions: false,
});

Code changes:

• Changed src/server/auth-client.ts: Added logic to delete transaction cookies after a callback is handled (on success or error).
• Changed src/server/client.ts: Plumbed the enableParallelTransactions option through to the TransactionStore.
• Changed src/server/transaction-store.ts: Implemented the logic to handle both single and parallel transaction modes.

Test changes:

• Changed src/server/auth-client.test.ts: Updated test mocks for new method signatures.
• Changed src/server/client.test.ts: Added tests to verify enableParallelTransactions is passed correctly.
• Changed src/server/redundant-txn-cookie-deletion.test.ts: Added an integration test suite with msw to verify the fix for the cookie accumulation bug.

Docs changes:

• Changed EXAMPLES.md: Added a "Transaction Cookie Configuration" section to document the new feature.
• Changed README.md: Updated the configuration options table with enableParallelTransactions.
• Changed V4_MIGRATION_GUIDE.md: Added a section on the new transaction cookie management.

📎 References

#1917
#2209

This PR aims to provide a possible solution to the transaction cookie pileup problem identified in the above issues. While not a complete solution, this can help bridge the gap b/w v3 and v4 behaviour.

Transaction cookies are created everytime startInteractiveLogin (called by handleLogin) redirects for login. These cookies are deleted once the callback handler handleCallback either succeeds or throws and error(added in this change) for the specific transaction state for which it was called.

These transaction cookies are NOT deleted when the callback handler is never called (the user navigates away from the login form (user manually doing this, or a misconfigured setup that ends up in an infinite redirect), in which case the cookies get automatically deleted a/c to their maxAge (this is 1hr by default, another PR will enable configuring this). If enough of these login attempts are cancelled before the txn cookie maxage, cookie storage can fill up.

By using a enableParallelTransactions = false, a single transaction cookie will be used per browser window. If one login transaction is active, trying to login again will fail with a warning.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.18519% with 8 lines in your changes missing coverage. Please review.

Project coverage is 85.23%. Comparing base (2df00f2) to head (ee2b696).

Files with missing lines	Patch %	Lines
src/server/transaction-store.ts	66.66%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2244      +/-   ##
==========================================
- Coverage   85.26%   85.23%   -0.04%     
==========================================
  Files          26       26              
  Lines        2409     2451      +42     
  Branches      449      460      +11     
==========================================
+ Hits         2054     2089      +35     
- Misses        349      356       +7     
  Partials        6        6              

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