fix: resolve event loop isolation issues in test suite

- Add session-scoped event loop fixture to prevent 'Event loop is closed' errors
- Use NullPool for database connections to avoid asyncpg connection caching issues
- Override session.commit with flush in tests to maintain transaction rollback
- Configure pytest-asyncio with session-scoped loop defaults
- Fixes 'coroutine Connection._cancel was never awaited' warnings
- Properly dispose of database engines after each test

Results: 137 tests passing (up from 116), only 8 failures remaining
This addresses the SQLAlchemy 2.0 async session lifecycle issues with asyncpg

server/pyproject.toml

@@ -117,6 +117,7 @@ DATABASE_URL = "postgresql+asyncpg://test_user:test_password@localhost:15432/ref
 addopts = "-ra -q --disable-pytest-warnings --cov --cov-report html -v"
 testpaths = ["tests"]
 asyncio_mode = "auto"
+asyncio_default_fixture_loop_scope = "session"
 markers = [
     "model_api: tests for the unified model-serving HTTP API (backend- and hardware-agnostic)",
 ]