impl(oauth2): add BackgroundCredentials decorator and update tests

[scotthart]

This PR moves the BackgroundThreads member from the RABTokenManager to a parent decorator, and the RABTokenManager is constructed with a CompletionQueue from the parent decorator BackgroundThreads.

This is all necessary to avoid a situation where a thread in the pool could attempt to destroy the BackgroundThreads object. As ~BackgroundThreads joins all threads in the pool, this creates an opportunity for a thread in the pool to attempt to join itself, which is an error.

The GCS test is also updated to eliminate flakiness introduced by the RAB functionality.

[gemini-code-assist]

Code Review

This pull request introduces BackgroundCredentials, a decorator class designed to manage the lifetime of BackgroundThreads used by child credentials, preventing thread join-on-self issues. It refactors RegionalAccessBoundaryTokenManager to accept a completion queue (RestPureCompletionQueue) instead of managing its own background threads, and updates associated tests and integration tests accordingly. Feedback was provided regarding BackgroundCredentials::project_id(Options const&), which currently ignores the passed Options parameter and should instead delegate to the child's overload that accepts options.

[codecov]

Codecov Report

❌ Patch coverage is 89.28571% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.20%. Comparing base (e908d88) to head (5ec2e29).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...gle/cloud/internal/oauth2_background_credentials.h	72.72%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16159      +/-   ##
==========================================
- Coverage   92.20%   92.20%   -0.01%     
==========================================
  Files        2264     2265       +1     
  Lines      209084   209113      +29     
==========================================
+ Hits       192791   192815      +24     
- Misses      16293    16298       +5     

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[alvarowolfx]

It seems good to me, just not sure about what the AI found on passing the Options on project_id call. Some questions that I added might be related to my poor knowledge of async stuff in CPP