Fix FastAPI lifespan integration in pytest

[Zaczero]

Describe the bug

Currently because of broken lifespan integration in pytest, you need to duplicate code like:

@pytest_asyncio.fixture(scope="session")
async def app_client(app):
    await create_index_templates()
    await create_collection_index()

    async with AsyncClient(
        transport=ASGITransport(app=app), base_url="http://test-server"
    ) as c:
        yield c

despite having:

@asynccontextmanager
async def lifespan(app: FastAPI):
    """Lifespan handler for FastAPI app. Initializes index templates and collections at startup."""
    await create_index_templates()
    await create_collection_index()
    yield

It's fairly simple to fix, feel free to use this code:

• https://github.com/openstreetmap-ng/openstreetmap-ng/blob/main/tests/utils/lifespan_manager.py
• https://github.com/openstreetmap-ng/openstreetmap-ng/blob/2ae47ba66659a4993a78acffa0a7126f95f87abc/tests/conftest.py#L51-L59

This should make the tests more consistent with the actual deployment.

[Zaczero]

This issue may be irrelevant! Depends if #394 is intended behavior or not.