test: consolidate SQLSpec test contracts

[cofin]

Summary

• Consolidates repeated migration, storage, event queue, framework, and adapter test families into explicit contract suites.
• Revises the test consolidation scope so broad adapter-local holdouts are not the endpoint: shared adapter capabilities now move into every-applicable-adapter contract matrices.
• Adds tests for storage bridge and exception mapping across SQLite, MySQL async, Postgres-family, ADBC, DuckDB, Oracle, Spanner, and existing BigQuery emulator-gated cases.
• Keeps the existing dependency lock refresh commit on this branch.

Validation

• uv run pytest tests/integration/adapters/contracts/test_storage_bridge.py -q -> 14 passed
• uv run pyright tests/integration/adapters/contracts/test_storage_bridge.py -> 0 errors
• uv run pytest tests/integration/adapters/contracts/test_exceptions.py -q -> 77 passed, 3 skipped
• uv run pyright tests/integration/adapters/contracts/test_exceptions.py -> 0 errors
• uv run pytest tests/integration/adapters --collect-only -q -> 175 files / 2104 items
• uvx prek run --files tests/integration/adapters/contracts/test_storage_bridge.py tests/integration/adapters/contracts/README.md

Beads: sqlspec-wp2 closed; C6 follow-up sqlspec-wp2.6 is active for the revised every-adapter scope.