test(bigquery): add integration tests for pending stream

[haphungw]

Add integration tests for the Pending write streams introduced in #6474.
Refactor the test suites for bigquery write.

For #6402

[gemini-code-assist]

Code Review

This pull request refactors the BigQuery write tests by introducing a dedicated WriteUserRecord struct and helper functions to set up and verify write operations. It updates the basic write test to use these helpers and adds a new pending write test to verify pending stream writes. The feedback identifies a redundant import in tests/bigquery/src/writes/arrow.rs that can be removed since the items are already imported via use super::*;.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.37%. Comparing base (18ffab7) to head (0ce67f2).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6478   +/-   ##
=======================================
  Coverage   96.37%   96.37%           
=======================================
  Files         297      297           
  Lines       84017    84017           
=======================================
+ Hits        80971    80973    +2     
+ Misses       3046     3044    -2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.