feat: create a generic wrapper for gRPC and REST pure background thre…

[bajajneha27]

…ad impl

[gemini-code-assist]

Code Review

This pull request refactors the background thread creation logic by introducing a generic template class, AutomaticallyCreatedBackgroundThreadsImpl, to consolidate duplicate code from both gRPC and REST background thread implementations. However, the reviewer identified a critical issue where this generic implementation breaks REST pure background threads due to invalid default-construction of RestPureCompletionQueue and the omission of internal::CallContext setup. To resolve this, the reviewer suggested introducing a customizable QueueTraits template parameter to handle queue-specific initialization and execution logic.

[codecov]

Codecov Report

❌ Patch coverage is 95.94595% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.21%. Comparing base (e908d88) to head (b36200c).

Files with missing lines	Patch %	Lines
...e/cloud/internal/generic_background_threads_impl.h	89.65%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16154   +/-   ##
=======================================
  Coverage   92.20%   92.21%           
=======================================
  Files        2264     2264           
  Lines      209084   209097   +13     
=======================================
+ Hits       192791   192813   +22     
+ Misses      16293    16284    -9     

☔ 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.