feat: add cloud job session controls

[cofin]

Summary

• keep BigQuery job retry and timeout controls on the existing query/load/export surfaces instead of adding new public helpers
• route BigQuery export coverage through the existing select_to_storage path
• keep Spanner request options, directed reads, retry, and timeout on execute(), execute_many(), execute_script(), and explicit provide_session()/provide_read_session()/provide_write_session() arguments
• remove the extra Spanner database-operation surface that was outside the existing driver contract

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.16667% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.98%. Comparing base (a876f40) to head (0dbdf2a).

Files with missing lines	Patch %	Lines
sqlspec/adapters/bigquery/core.py	60.71%	4 Missing and 7 partials ⚠️
sqlspec/adapters/spanner/adk/store.py	30.76%	8 Missing and 1 partial ⚠️
sqlspec/adapters/spanner/driver.py	90.76%	3 Missing and 3 partials ⚠️
sqlspec/adapters/spanner/config.py	76.47%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #523      +/-   ##
==========================================
+ Coverage   74.87%   74.98%   +0.10%     
==========================================
  Files         441      441              
  Lines       55085    55213     +128     
  Branches     8689     8721      +32     
==========================================
+ Hits        41245    41399     +154     
+ Misses      11156    11109      -47     
- Partials     2684     2705      +21     

Flag	Coverage Δ
integration	58.58% <27.77%> (-0.09%)	⬇️
py3.10	71.86% <75.00%> (+0.11%)	⬆️
py3.11	71.88% <75.00%> (+0.11%)	⬆️
py3.12	71.88% <75.00%> (+0.11%)	⬆️
py3.13	71.88% <75.00%> (+0.11%)	⬆️
py3.14	74.11% <76.59%> (+0.11%)	⬆️
unit	62.08% <72.22%> (+0.21%)	⬆️

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

Files with missing lines	Coverage Δ
sqlspec/adapters/bigquery/config.py	87.58% <100.00%> (+0.08%)	⬆️
sqlspec/adapters/bigquery/driver.py	80.97% <100.00%> (+12.00%)	⬆️
sqlspec/adapters/spanner/config.py	87.35% <76.47%> (-0.89%)	⬇️
sqlspec/adapters/spanner/driver.py	76.66% <90.76%> (+4.49%)	⬆️
sqlspec/adapters/spanner/adk/store.py	62.05% <30.76%> (-1.08%)	⬇️
sqlspec/adapters/bigquery/core.py	69.48% <60.71%> (+6.73%)	⬆️

... and 3 files with indirect coverage changes

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