fix(bigtable): treat NOT_FOUND and PERMISSION_DENIED on channel refresh as success

[scotthart]

Primarily an implementation detail, Bigtable's DynamicChannelPool feature calls the PingAndWarm RPC to prime and refresh channels. Even if this RPC fails due a permission configuration issue, it still indicates a successful path to the service was setup.

[gemini-code-assist]

Code Review

This pull request updates the Bigtable client to treat kPermissionDenied and kNotFound as acceptable status codes during connection refreshes, modifying logging in bigtable_stub_factory.cc, RPC tracking in channel_usage.h, and adding corresponding test cases. Review feedback recommends centralizing the status-checking logic to avoid duplication and using the .ok() method for more idiomatic and defensive status validation.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.70%. Comparing base (d2e9ba4) to head (3a1cc50).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/cloud/bigtable/internal/bigtable_stub_factory.cc	0.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16086   +/-   ##
=======================================
  Coverage   92.69%   92.70%           
=======================================
  Files        2351     2351           
  Lines      218102   218117   +15     
=======================================
+ Hits       202178   202205   +27     
+ Misses      15924    15912   -12     

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