feat: add Arrow ODBC SQL Server contract coverage

[cofin]

Summary

• Enable Arrow ODBC sync coverage in the shared adapter contract matrix against SQL Server 2022.
• Add adapter-local Arrow ODBC extension backends for events, Litestar sessions, ADK sessions, and ADK memory without cross-adapter imports.
• Update CI to install Microsoft ODBC Driver 18 and pre-pull the SQL Server 2022 image, and refresh the related docs.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 69.70199% with 183 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.65%. Comparing base (cc562c3) to head (cb874e3).

Files with missing lines	Patch %	Lines
sqlspec/adapters/arrow_odbc/adk/store.py	59.83%	125 Missing and 18 partials ⚠️
sqlspec/adapters/arrow_odbc/litestar/store.py	82.06%	15 Missing and 11 partials ⚠️
sqlspec/adapters/arrow_odbc/events/store.py	77.55%	5 Missing and 6 partials ⚠️
sqlspec/adapters/arrow_odbc/config.py	75.00%	1 Missing and 1 partial ⚠️
sqlspec/adapters/arrow_odbc/driver.py	95.83%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
- Coverage   75.68%   75.65%   -0.03%     
==========================================
  Files         449      455       +6     
  Lines       59986    60584     +598     
  Branches     9144     9221      +77     
==========================================
+ Hits        45398    45836     +438     
- Misses      11657    11783     +126     
- Partials     2931     2965      +34     

Flag	Coverage Δ
integration	60.06% <69.20%> (+0.69%)	⬆️
py3.10	73.88% <50.99%> (-0.20%)	⬇️
py3.11	73.90% <50.99%> (-0.20%)	⬇️
py3.12	73.89% <50.99%> (-0.20%)	⬇️
py3.13	73.89% <50.99%> (-0.20%)	⬇️
py3.14	74.85% <69.65%> (-0.02%)	⬇️
unit	61.28% <7.11%> (-0.54%)	⬇️

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

Files with missing lines	Coverage Δ
sqlspec/adapters/arrow_odbc/adk/__init__.py	100.00% <100.00%> (ø)
sqlspec/adapters/arrow_odbc/core.py	86.76% <100.00%> (+0.61%)	⬆️
sqlspec/adapters/arrow_odbc/events/__init__.py	100.00% <100.00%> (ø)
sqlspec/adapters/arrow_odbc/litestar/__init__.py	100.00% <100.00%> (ø)
sqlspec/extensions/events/_queue.py	88.42% <100.00%> (+0.49%)	⬆️
sqlspec/adapters/arrow_odbc/driver.py	70.80% <95.83%> (+10.71%)	⬆️
sqlspec/adapters/arrow_odbc/config.py	82.78% <75.00%> (+0.17%)	⬆️
sqlspec/adapters/arrow_odbc/events/store.py	77.55% <77.55%> (ø)
sqlspec/adapters/arrow_odbc/litestar/store.py	82.06% <82.06%> (ø)
sqlspec/adapters/arrow_odbc/adk/store.py	59.83% <59.83%> (ø)

... and 2 files with indirect coverage changes

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