fix(providers): enable graceful termination during SSO browser authentication

[8nevil8]

Summary

Fixes issue where SSO authentication flow cannot be interrupted with Ctrl+C when waiting for browser callback. Previously, the process would hang indefinitely, requiring manual termination (kill -9).

Changes

• Added AbortController support to enable external cancellation of authentication flow
• Registered SIGINT/SIGTERM handlers following the pattern from agent.ts:486-496 for consistent signal handling across the codebase
• Replaced recursive polling in waitForCallback with non-recursive setInterval to prevent uninterruptible wait loops
• Enhanced cleanup() method to force-close HTTP server connections immediately using closeAllConnections()
• Added proper error handling for user cancellation with clear error messages
• Implemented proper resource cleanup including timer cleanup and state reset

Impact

Before:

1. Browser opens for SSO authentication
2. User presses Ctrl+C
3. ❌ Process hangs indefinitely
4. ❌ Requires manual kill -9

After:

1. Browser opens for SSO authentication
2. User presses Ctrl+C
3. ✅ Shows "⚠️  Authentication cancelled by user"
4. ✅ Server closes immediately
5. ✅ Process terminates gracefully

Checklist

• Self-reviewed
• Manual testing performed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)