fix(adk): repair PostgreSQL vector memory search (#726)

[cofin]

Summary

PostgreSQL ADK memory stores omitted persisted embeddings and bound vector operands in a form that required optional pgvector client codecs. BM25 capability detection also relied on pg_search although the index and operator require pg_textsearch.

This persists nullable embeddings and casts PostgreSQL vector bindings through float8[], applies the cast to vector and hybrid searches, and validates cached pg_textsearch capability before BM25 DDL or queries.

Fixes #726

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.28916% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.08%. Comparing base (6a7433d) to head (cd38499).

Files with missing lines	Patch %	Lines
sqlspec/adapters/psycopg/config.py	73.33%	5 Missing and 3 partials ⚠️
sqlspec/adapters/asyncmy/adk/store.py	14.28%	6 Missing ⚠️
sqlspec/adapters/psycopg/adk/store.py	33.33%	2 Missing and 2 partials ⚠️
sqlspec/adapters/asyncpg/config.py	80.00%	1 Missing and 2 partials ⚠️
sqlspec/adapters/asyncpg/adk/store.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #727      +/-   ##
==========================================
+ Coverage   76.95%   77.08%   +0.13%     
==========================================
  Files         487      487              
  Lines       69755    69819      +64     
  Branches     9662     9676      +14     
==========================================
+ Hits        53677    53819     +142     
+ Misses      12643    12541     -102     
- Partials     3435     3459      +24     

Flag	Coverage Δ
integration	60.60% <43.37%> (+0.09%)	⬆️
py3.10	75.35% <69.87%> (+0.13%)	⬆️
py3.11	75.36% <69.87%> (+10.70%)	⬆️
py3.12	75.38% <69.87%> (+0.14%)	⬆️
py3.13	75.37% <69.87%> (+0.14%)	⬆️
py3.14	76.28% <69.87%> (+0.12%)	⬆️
unit	65.14% <68.67%> (+0.15%)	⬆️

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

Files with missing lines	Coverage Δ
sqlspec/adapters/asyncmy/config.py	90.59% <100.00%> (ø)
sqlspec/adapters/asyncmy/litestar/store.py	80.46% <100.00%> (+0.15%)	⬆️
sqlspec/config.py	93.92% <100.00%> (ø)
...xtensions/adk/migrations/0001_create_adk_tables.py	80.80% <100.00%> (+2.15%)	⬆️
sqlspec/adapters/asyncpg/adk/store.py	64.37% <60.00%> (+3.19%)	⬆️
sqlspec/adapters/asyncpg/config.py	93.20% <80.00%> (+0.77%)	⬆️
sqlspec/adapters/psycopg/adk/store.py	61.02% <33.33%> (+9.30%)	⬆️
sqlspec/adapters/asyncmy/adk/store.py	59.54% <14.28%> (+0.09%)	⬆️
sqlspec/adapters/psycopg/config.py	89.97% <73.33%> (-0.63%)	⬇️

... and 5 files with indirect coverage changes

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