Prevent stuck cancel requests from blocking close

[aryansk]

Summary

Prevent an in-flight PostgreSQL CancelRequest task from keeping Connection.close(timeout=...) alive beyond the requested graceful-close timeout.

Changes

• cancel outstanding background cancellation tasks before beginning the graceful protocol close
• rely on the existing cancellation-task cleanup path to discard those tasks and resolve their waiters
• leave normal connection close behavior unchanged when no cancellation is pending

Test plan

Use the self-contained fake-server reproducer from #1326: after a command timeout creates a held-open cancel connection, conn.close(timeout=2) should no longer wait indefinitely.

Fixes #1326