fix(bigquery): stop leaking gRPC streams, stream tasks

[dbolduc]

This was a legit bug. We were leaking open streams and the tasks running them when the application finished writing.

Fixes #6715

[gemini-code-assist]

Code Review

This pull request modifies the BigQuery write runner to explicitly drop the request_tx sender when the request receiver (req_rx) is closed (returns None), ensuring the stream is properly closed. It also adds a corresponding integration test stream_closes_when_client_drops_sender to verify this behavior. I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.72%. Comparing base (8927911) to head (17f36c9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6729   +/-   ##
=======================================
  Coverage   96.71%   96.72%           
=======================================
  Files         313      313           
  Lines      101265   101290   +25     
=======================================
+ Hits        97938    97969   +31     
+ Misses       3327     3321    -6     

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