chore: default to pytest asyncio-mode=auto (#2730)

# What does this PR do?

previously, developers who ran `./scripts/unit-tests.sh` would get
`asyncio-mode=auto`, which meant `@pytest.mark.asyncio` and
`@pytest_asyncio.fixture` were redundent. developers who ran `pytest`
directly would get pytest's default (strict mode), would run into errors
leading them to add `@pytest.mark.asyncio` / `@pytest_asyncio.fixture`
to their code.

with this change -
- `asyncio_mode=auto` is included in `pyproject.toml` making behavior
consistent for all invocations of pytest
- removes all redundant `@pytest_asyncio.fixture` and
`@pytest.mark.asyncio`
 - for good measure, requires `pytest>=8.4` and `pytest-asyncio>=1.0`

## Test Plan

- `./scripts/unit-tests.sh`
- `uv run pytest tests/unit`

tests/unit/server/test_auth.py

@@ -202,7 +202,6 @@ def test_http_auth_request_payload(http_client, valid_api_key, mock_auth_endpoin
         assert "param2" in payload["request"]["params"]
 
 
-@pytest.mark.asyncio
 async def test_http_middleware_with_access_attributes(mock_http_middleware, mock_scope):
     """Test HTTP middleware behavior with access attributes"""
     middleware, mock_app = mock_http_middleware