fix: cleanup OAuth event listeners and interval on login success

[Anshumancanrock]

Fix: OAuth event listener and interval memory leak

Acceptance Criteria fulfillment

• Event listener and interval are cleaned up immediately after successful OAuth login
• Cleanup occurs when user closes popup without completing login
• No race conditions when message and popup close happen simultaneously
• API call errors are properly handled and rejected
• Memory leaks eliminated for repeated OAuth authentication flows

Fixes #1125

Description

This PR fixes a memory leak in the OAuth authentication flow where event listeners and interval timers persisted after login completion.

Before:

Event listeners and intervals weren't cleaned up on login success, causing memory leaks and hanging promises when popup was closed manually.

After:

Added cleanup() function with settled flag to immediately remove listeners/intervals on success or popup close, preventing race conditions and properly handling API errors.

PR Test Details

Testing OAuth flow:

1. Enable OAuth authentication in your EmbeddedChat config
2. Click login button to trigger OAuth flow
3. Complete login successfully → verify no console errors and authentication works
4. Repeat login multiple times → check browser's event listeners don't accumulate
5. Close popup without logging in → verify proper error message is displayed

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[Anshumancanrock]

hi @Spiral-Memory , Could you please review this PR? Thanks !