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/integration/providers/utils/sqlstore/test_authorized_sqlstore.py

@@ -88,7 +88,6 @@ async def cleanup_records(sql_store, table_name, record_ids):
             pass
 
 
-@pytest.mark.asyncio
 @pytest.mark.parametrize("backend_config", BACKEND_CONFIGS)
 @patch("llama_stack.providers.utils.sqlstore.authorized_sqlstore.get_authenticated_user")
 async def test_authorized_store_attributes(mock_get_authenticated_user, authorized_store, request):
@@ -183,7 +182,6 @@ async def test_authorized_store_attributes(mock_get_authenticated_user, authoriz
         await cleanup_records(authorized_store.sql_store, table_name, ["1", "2", "3", "4", "5", "6"])
 
 
-@pytest.mark.asyncio
 @pytest.mark.parametrize("backend_config", BACKEND_CONFIGS)
 @patch("llama_stack.providers.utils.sqlstore.authorized_sqlstore.get_authenticated_user")
 async def test_user_ownership_policy(mock_get_authenticated_user, authorized_store, request):