fix: use ASGITransport for httpx AsyncClient in tests

- httpx.AsyncClient requires ASGITransport for FastAPI app testing
- Updated all test files to use correct API

backend/tests/conftest.py

@@ -3,7 +3,7 @@ import pytest_asyncio
 from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession, async_sessionmaker
 from app.database import Base, get_db
 from app.main import app
-from httpx import AsyncClient
+from httpx import AsyncClient, ASGITransport
 
 TEST_DATABASE_URL = "sqlite+aiosqlite:///:memory:"
 
@@ -28,6 +28,6 @@ async def client(db):
         yield db
     
     app.dependency_overrides[get_db] = override_get_db
-    async with AsyncClient(app=app, base_url="http://test") as ac:
+    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as ac:
         yield ac
     app.dependency_overrides.clear()