This PR primarily fixes#572 by enabling graceful shutdown without consuming self. While implementing this, I noticed delete_session() is spawned as a background task, which means close() may return before HTTP session cleanup completes. Since this is part of the same shutdown lifecycle and can cause resource leaks/races, I'm including a small, localized fix to ensure cleanup is completed before close() returns. If maintainers prefer, I can split the cleanup timing change into a follow-up PR.
Changes:
- Add close(&mut self) for graceful shutdown without consuming
- Add close_with_timeout() for bounded shutdown operations
- Add is_closed() to check connection state
- Move HTTP delete_session from background spawn to inline cleanup
- Add 5-second timeout on session cleanup to prevent indefinite hangs
- Add Drop impl with debug log if dropped without explicit close
Fixes#572
