chore(bigtable): fix some formatting and auto issues

[scotthart]

Cleans up some full explicit namespace usage where it wasn't needed as well as switching some uses of auto to explicit types.

[gemini-code-assist]

Code Review

This pull request simplifies namespace usage in the BigtableRandomTwoLeastUsed decorator by removing redundant google::cloud:: prefixes and updates stub_manager_test.cc to use explicit OperationContext& types instead of auto const& in lambda parameters. The review feedback suggests using void() instead of void(void) for empty parameter lists to align with the Google C++ style guide, and passing std::function parameters by const& instead of by value in helper functions to avoid unnecessary copy overhead.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.25%. Comparing base (5a449d9) to head (b86c2dc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16369      +/-   ##
==========================================
- Coverage   92.27%   92.25%   -0.02%     
==========================================
  Files        2239     2239              
  Lines      210719   210719              
==========================================
- Hits       194444   194406      -38     
- Misses      16275    16313      +38     

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