fix(bigquery): avoid panic on idle stream shutdown

[dbolduc]

The server always sends one response per request... but it can also end the stream in an error without any requests in the queue. We need to account for this.

Fixes #6815

[gemini-code-assist]

Code Review

This pull request updates the BigQuery write runner to gracefully handle idle stream closures when no requests are queued, preventing panics from popping an empty response queue. It also adds a regression test to verify this behavior. The feedback suggests refactoring the conditional block using a let-else statement to return early, which simplifies the logic and aligns with the repository's style guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.03%. Comparing base (bcefaf4) to head (283c51e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6816   +/-   ##
=======================================
  Coverage   97.02%   97.03%           
=======================================
  Files         326      326           
  Lines      107548   107569   +21     
=======================================
+ Hits       104348   104375   +27     
+ Misses       3200     3194    -6     

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