test: add API endpoint test suite and enable the CI test job

[gordonmurray]

Summary

The CI test job has been printing debug output since the endpoint tests were commented out. The blocker turned out to be a dependency clash: the job installs unpinned httpx, and httpx 0.28 removed the app= kwarg that the starlette version pinned by fastapi 0.104 still passes to TestClient. Pinning httpx<0.28 fixes that, so the job now runs a real pytest suite.

• 30 tests in backend/tests/ using pytest and FastAPI TestClient: health and compat flags, dataset listing, schema and column metadata, pagination and column filtering, value serialization (nulls, binary, vectors, CLIP detection), vector preview stats, and the informational-row fallback for corrupted datasets.
• The fixture builds sample tables in a tmpdir with whatever lancedb version is installed, so the same tests run for every constraints file. One quirk it handles: Lance format v1 (0.3.x/0.5) stores a null list as an empty list, so the affected tests check at runtime which behavior the installed version has.
• One small backend fix to make the tests meaningful: /rows and /vector/preview re-raise HTTPException, so the 400 for unknown column names reaches the client instead of being turned into a generic 500 by the catch-all handler.

Test plan

• Ran the suite in python:3.11-slim against all six constraints files (0.3.1, 0.3.4, 0.5, 0.16.0, 0.24.3, 0.29.2): 30 passed on each
• CI test job goes green on this PR for the full matrix

Fixes #28