feat: add adapter type-system release groundwork

[cofin]

Summary

• Add disabled-first adapter type-system contract scaffolding and source-equivalence tripwires.
• Optimize shared schema conversion, serializer metadata caching, row-format checks, row-format assertions, and benchmark coverage.
• Switch Oracle JSON output handling to driver metadata and make ordinary CLOB/BLOB fetches metadata-driven rather than content-sniffed.
• Align Oracle LOB fetch behavior across buffered, streamed, Arrow, and async cached paths with fetch_lobs=False as the default and fetch_lobs=True as the explicit locator escape hatch.
• Add Spanner metadata-driven row plans and native row streaming with buffered/streamed parity and preserved per-call execution options.
• Move Cloud/Arrow hot paths toward native row materialization: ADBC Arrow-table rows, BigQuery stream row mappings, and serializer hooks for Arrow ODBC/MSSQL configs.
• Align PG-family and embedded adapters with single-pass parameter handling, row-format detection, faster aiosqlite/duckdb paths, and explicit Cockroach transaction-level retry callbacks.
• Remove dead output converter cache paths and obsolete adapter converter helpers while preserving live conversion surfaces.

Local verification

• uv run ruff check on touched adapter/test slices and file-level uvx prek for changed files.
• Focused unit and integration slices for Oracle JSON/LOB behavior, Spanner row plans/streaming, schema conversion, serializers/type guards, adapter converter cleanup, Cloud/Arrow row conversion, PG/embedded adapter behavior, and Cockroach transaction retry behavior.
• uv run mypy
• uv run pyright
• SQLite extended benchmark smoke with schema_type_numpy scenario.
• Aiosqlite extended benchmark smoke with worker_hops scenario.

Full matrix, long mypyc compilation, and service-backed integration checks are left to remote CI.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.32422% with 257 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.70%. Comparing base (2a1daf1) to head (f29831b).

Files with missing lines	Patch %	Lines
sqlspec/adapters/aiosqlite/driver.py	57.69%	37 Missing and 18 partials ⚠️
sqlspec/adapters/mssql_python/driver.py	79.16%	16 Missing and 9 partials ⚠️
sqlspec/adapters/duckdb/driver.py	67.12%	13 Missing and 11 partials ⚠️
sqlspec/adapters/oracledb/driver.py	77.02%	10 Missing and 7 partials ⚠️
sqlspec/adapters/spanner/driver.py	86.51%	7 Missing and 5 partials ⚠️
sqlspec/adapters/adbc/driver.py	82.53%	6 Missing and 5 partials ⚠️
sqlspec/adapters/aiomysql/core.py	77.27%	5 Missing and 5 partials ⚠️
sqlspec/adapters/asyncmy/core.py	77.27%	5 Missing and 5 partials ⚠️
sqlspec/adapters/mysqlconnector/core.py	82.75%	5 Missing and 5 partials ⚠️
sqlspec/adapters/pymysql/core.py	76.74%	5 Missing and 5 partials ⚠️
... and 22 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #603      +/-   ##
==========================================
+ Coverage   75.57%   75.70%   +0.13%     
==========================================
  Files         474      471       -3     
  Lines       62126    62720     +594     
  Branches     9309     9424     +115     
==========================================
+ Hits        46951    47483     +532     
+ Misses      12164    12145      -19     
- Partials     3011     3092      +81     

Flag	Coverage Δ
integration	60.11% <55.02%> (+0.94%)	⬆️
py3.10	73.96% <78.11%> (+0.12%)	⬆️
py3.11	73.98% <78.27%> (+0.13%)	⬆️
py3.12	73.97% <78.11%> (+0.12%)	⬆️
py3.13	73.97% <78.27%> (+0.12%)	⬆️
py3.14	74.91% <78.92%> (+0.14%)	⬆️
unit	61.93% <69.34%> (+0.48%)	⬆️

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	92.72% <100.00%> (+0.04%)	⬆️
sqlspec/adapters/adbc/type_converter.py	76.92% <100.00%> (+23.79%)	⬆️
sqlspec/adapters/aiomysql/config.py	93.54% <100.00%> (-0.07%)	⬇️
sqlspec/adapters/aiosqlite/core.py	90.09% <100.00%> (-0.28%)	⬇️
sqlspec/adapters/bigquery/core.py	72.11% <100.00%> (+0.35%)	⬆️
sqlspec/adapters/duckdb/core.py	85.52% <100.00%> (+0.91%)	⬆️
sqlspec/adapters/mysqlconnector/config.py	94.73% <100.00%> (+0.06%)	⬆️
sqlspec/adapters/oracledb/_typing.py	92.92% <ø> (ø)
sqlspec/adapters/oracledb/type_converter.py	92.85% <100.00%> (+8.24%)	⬆️
sqlspec/adapters/psqlpy/type_converter.py	66.66% <100.00%> (+15.05%)	⬆️
... and 42 more

... and 5 files with indirect coverage changes

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