Fix: Use a short-live session instead of passing sessionDeps into DagRunWaiter

[sjyangkevin]

related: #64415

Why

The SessionDep session is held open from the moment the endpoint function starts until the StreamingResponse generator is fully consumed and FastAPI cleans up the dependency. During the polling loop (while dag_run.state not in State.finished_dr_states: await asyncio.sleep(self.interval)), the sync DB connection sits idle but checked out from the pool. If a DAG run takes minutes or hours to complete, that connection is locked for the entire duration. With many concurrent /wait requests, this can exhaust the connection pool.

This PR improves the above by opening a short-live session in _serialize_xcoms, similar to what have been done in _get_dag_run.

#64415 (comment)

Test Cases

Tests should have been handled in test_collect_task and test_should_respond_200_immediately_for_finished_run

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