cleanup(bigquery): centralize test helpers

[dbolduc]

We have a bunch of test helpers defined at wherever I first needed them. Move them all to a centralized mod test, so they are easier to find.

[gemini-code-assist]

Code Review

This pull request refactors the BigQuery write client's unit tests by consolidating common test helper functions—such as 'write_stream', 'schema', 'test_transport', 'convert', 'test_request', and 'test_response'—into a new dedicated test helper module ('src/bigquery/src/write/test.rs'). Duplicate helper definitions across various test modules have been removed and replaced with imports from the new module, reducing boilerplate and improving maintainability. I have no feedback to provide as the changes are clean and idiomatic.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.71%. Comparing base (9797b2d) to head (c7c7c6e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6730      +/-   ##
==========================================
- Coverage   96.71%   96.71%   -0.01%     
==========================================
  Files         313      314       +1     
  Lines      101324   101293      -31     
==========================================
- Hits        98000    97965      -35     
- Misses       3324     3328       +4     

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