feat(psycopg): add AlloyDB connector support

[cofin]

Summary

• add opt-in sync AlloyDB Connector support for PsycopgSyncConfig
• inject a connector-backed psycopg connection class into psycopg_pool.ConnectionPool
• document the sync-only connector path and cover validation, routing, database-key mapping, async isolation, and cleanup with unit tests

Tests

• uv run pytest tests/unit/adapters/test_psycopg -q --tb=short
• uv run ruff check sqlspec/adapters/psycopg/config.py tests/unit/adapters/test_psycopg/test_cloud_connectors.py
• uv run ruff format --check sqlspec/adapters/psycopg/config.py tests/unit/adapters/test_psycopg/test_cloud_connectors.py

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.16129% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.70%. Comparing base (b4a7d05) to head (d8dd89c).

Files with missing lines	Patch %	Lines
sqlspec/adapters/psycopg/config.py	95.16%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #568      +/-   ##
==========================================
+ Coverage   75.68%   75.70%   +0.02%     
==========================================
  Files         449      449              
  Lines       59986    60044      +58     
  Branches     9144     9156      +12     
==========================================
+ Hits        45398    45457      +59     
+ Misses      11657    11655       -2     
- Partials     2931     2932       +1     

Flag	Coverage Δ
integration	59.34% <30.64%> (-0.03%)	⬇️
py3.10	74.10% <91.93%> (+0.02%)	⬆️
py3.11	74.11% <91.93%> (+0.02%)	⬆️
py3.12	74.11% <91.93%> (+0.02%)	⬆️
py3.13	74.11% <91.93%> (+0.02%)	⬆️
py3.14	74.89% <91.37%> (+0.01%)	⬆️
unit	61.85% <90.32%> (+0.03%)	⬆️

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Files with missing lines	Coverage Δ
sqlspec/adapters/psycopg/config.py	88.60% <95.16%> (+1.91%)	⬆️

... and 1 file with indirect coverage changes

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