fix(spanner): modernize adapter config

[cofin]

Summary

Updates the Spanner adapter config to route current Google Cloud Spanner client, instance, database, session-pool, and request execution options.

Details

• Expands typed connection config for current client settings, directed reads, observability, transaction defaults, instance creation options, database role, encryption, drop protection, and proto descriptors.
• Replaces stale min_sessions behavior with the session-pool controls used by current Spanner pools, including size, target_size, default_timeout, labels, and max age.
• Rejects min_sessions early so unsupported pool configuration does not appear to work.
• Routes session labels from the legacy driver-feature location into pool config for compatibility.
• Adds retry and timeout driver features and forwards them to execute_sql(), execute_update(), and batch_update().
• Adds unit coverage for pool selection, client/instance/database kwarg routing, typed config coverage, and retry/timeout execution forwarding.

Review Notes

This branch is rebased onto the latest main; the changes are limited to Spanner config routing and the request execution kwargs needed to honor the new driver features.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.38202% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.44%. Comparing base (2f1a993) to head (6d4ee34).

Files with missing lines	Patch %	Lines
sqlspec/adapters/spanner/config.py	93.67%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #511      +/-   ##
==========================================
+ Coverage   71.63%   74.44%   +2.81%     
==========================================
  Files         439      439              
  Lines       53427    53490      +63     
  Branches     8460     8464       +4     
==========================================
+ Hits        38270    39819    +1549     
+ Misses      12654    11048    -1606     
- Partials     2503     2623     +120     

Flag	Coverage Δ
integration	58.30% <75.28%> (+3.64%)	⬆️
py3.10	71.23% <93.25%> (+9.73%)	⬆️
py3.11	71.26% <93.25%> (+0.03%)	⬆️
py3.12	71.25% <93.25%> (+9.73%)	⬆️
py3.13	71.25% <93.25%> (+9.74%)	⬆️
py3.14	73.55% <88.67%> (+12.83%)	⬆️
unit	61.79% <93.25%> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sqlspec/adapters/spanner/driver.py	72.16% <100.00%> (+0.67%)	⬆️
sqlspec/adapters/spanner/config.py	88.23% <93.67%> (+6.56%)	⬆️

... and 40 files with indirect coverage changes

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