Fix race condition in concurrent session deletion

[shamil2]

Fixes race condition in concurrent session deletion where multiple DELETE requests to /session/{id} return success but don't reliably persist all deletions.

Changes

• Add write lock to Storage.remove for thread safety
• Add session-level locking in Session.remove to prevent concurrent deletions

Testing

The fix addresses the root cause identified in issue #5517 by ensuring:

• Only one deletion process runs per session at a time
• All storage operations are properly synchronized
• Concurrent DELETE requests are handled atomically

Closes #5517