feat(postgres): add pg_textsearch dialect and modular extension probing

[cofin]

Description

This PR introduces native support for AlloyDB and AlloyDB Omni's BM25 full-text search (pg_textsearch extension), decouples PostgreSQL extension dialects, and modularizes runtime extension probing:

• Dialect & AST Parsing:
  • Adds PGTextSearch(Postgres) dialect registering custom <@> operator tokenization without colliding with PostgreSQL containment operators or requiring sqlglot upstream changes.
  • Decouples ParadeDB and PGVector dialects so both inherit independently from Postgres, resolving dialect hierarchy conflicts.
  • Registers entry point pg_textsearch in pyproject.toml and dialect mapping in sqlspec/core/splitter.py.
• Active Extension State:
  • Replaces rigid dialect-string promotion with active_extensions: set[str] tracking in driver_features.
  • Exports is_postgres_extension_active() and standardizes build_postgres_extension_probe_names().
• Adapter Integration:
  • Adds enable_pg_textsearch: bool = True across all PostgreSQL adapters (asyncpg, psycopg, adbc, psqlpy).
  • Unifies first-connection probing across adapters, replacing ad-hoc ADK memory probes.
• Documentation & Compilation:
  • Registers _pg_textsearch.py in mypyc exclusions and inventory boundary maps.
  • Updates Sphinx reference docs (docs/reference/dialects.rst, adapter docs) and changelog (docs/changelog.rst).

Verification

• make lint (passed: Prek, Ruff, Codespell, slotscheck, Actions audit)
• make type-check (passed: Mypy, Pyright 0 errors)
• make docs (passed: build succeeded)
• Unit tests: 2,645 passed across dialects, core, and all PostgreSQL adapters

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.51724% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.15%. Comparing base (8547d5d) to head (5192045).

Files with missing lines	Patch %	Lines
sqlspec/core/config_runtime.py	76.19%	2 Missing and 3 partials ⚠️
sqlspec/adapters/adbc/type_converter.py	0.00%	0 Missing and 2 partials ⚠️
sqlspec/adapters/psycopg/config.py	81.81%	2 Missing ⚠️
sqlspec/adapters/adbc/core.py	92.85%	0 Missing and 1 partial ⚠️
sqlspec/dialects/postgres/_generators.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #782      +/-   ##
==========================================
+ Coverage   76.96%   78.15%   +1.19%     
==========================================
  Files         487      488       +1     
  Lines       71929    72003      +74     
  Branches    10075    10084       +9     
==========================================
+ Hits        55361    56277     +916     
+ Misses      13149    12271     -878     
- Partials     3419     3455      +36     

Flag	Coverage Δ
integration	61.07% <67.24%> (+2.20%)	⬆️
py3.10	76.47% <89.65%> (+0.02%)	⬆️
py3.11	66.64% <89.65%> (+0.05%)	⬆️
py3.12	76.47% <89.65%> (+0.02%)	⬆️
py3.13	76.48% <89.65%> (+9.89%)	⬆️
py3.14	77.40% <89.28%> (+11.27%)	⬆️
unit	66.98% <89.65%> (+0.05%)	⬆️

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

Files with missing lines	Coverage Δ
sqlspec/adapters/adbc/config.py	93.22% <100.00%> (+0.44%)	⬆️
sqlspec/adapters/asyncpg/config.py	93.56% <100.00%> (+0.35%)	⬆️
sqlspec/adapters/asyncpg/core.py	79.09% <100.00%> (+0.08%)	⬆️
sqlspec/adapters/psqlpy/config.py	92.69% <100.00%> (+0.25%)	⬆️
sqlspec/adapters/psqlpy/core.py	76.60% <100.00%> (+0.04%)	⬆️
sqlspec/adapters/psycopg/core.py	92.80% <100.00%> (+0.02%)	⬆️
sqlspec/builder/_ddl.py	68.61% <100.00%> (+0.14%)	⬆️
sqlspec/core/splitter.py	80.47% <ø> (ø)
sqlspec/dialects/__init__.py	70.00% <100.00%> (+30.00%)	⬆️
sqlspec/dialects/postgres/__init__.py	100.00% <100.00%> (ø)
... and 9 more

... and 40 files with indirect coverage changes

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