fix: clear _tx_owner when resetting connection transaction state

_reset_connection() cleared _in_transaction but left _tx_owner pointing
to the stale task. This created inconsistent state on connections
returned to the pool after a dirty rollback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqliteclient/pool.py

@@ -182,6 +182,7 @@ async def _reset_connection(self, conn: DqliteConnection) -> bool:
             except Exception:
                 return False
             conn._in_transaction = False
+            conn._tx_owner = None
         return True
 
     async def _release(self, conn: DqliteConnection) -> None: