chore(spanner): add configuration builders for dynamic channel pool

[olavloite]

Adds fluent builder methods and constructor helpers for DynamicChannelPoolConfig and StaticChannelPoolConfig.

This provides a builder API to customize channel pool settings, such as initial/min/max channel bounds, scale-up and scale-down thresholds, error penalty parameters, and channel selection strategies.

[gemini-code-assist]

Code Review

This pull request adds documentation examples, builder methods, and tests for the static and dynamic channel pool configurations in the Spanner client, while marking several configuration structs and enums as #[non_exhaustive]. The review feedback suggests keeping the static_config helper method in the main module gated by #[cfg(test)] rather than moving its implementation entirely into the test module, which improves code discoverability and centralization.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request adds documentation examples, #[non_exhaustive] attributes, and builder/constructor methods for the Spanner channel pool configurations, along with corresponding unit tests. The feedback suggests replacing the fluent builder methods on the configuration structs with standard setter methods to maintain API consistency across the repository.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.95%. Comparing base (4afc819) to head (b69720b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6736    +/-   ##
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  Coverage   96.95%   96.95%            
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  Files         313      313            
  Lines      105118   105234   +116     
========================================
+ Hits       101916   102031   +115     
- Misses       3202     3203     +1     

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