fix: return 422 for invalid recall fact types

[tao943]

Summary

• raise OperationValidationError(status_code=422) for unsupported recall fact types
• preserve the existing validation detail through the HTTP exception mapping
• add engine-level and HTTP-level regression coverage

Root cause

MemoryEngine.recall_async raised a plain ValueError for unknown fact types. The recall HTTP handler maps OperationValidationError to its declared status code, but the plain ValueError fell through to the broad exception handler and became a 500 response.

Testing

• python -m pytest tests/test_recall_invalid_fact_types.py tests/test_http_api_integration.py::test_error_handling tests/test_http_api_integration.py::test_full_api_workflow -v -n 0 — 3 passed
• ruff check . — passed
• ruff format --check . — 582 files already formatted
• ty check hindsight_api/engine/memory_engine.py — passed

Closes #3052