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/rag/test_rag_query.py

@@ -17,13 +17,11 @@ from llama_stack.providers.inline.tool_runtime.rag.memory import MemoryToolRunti
 
 
 class TestRagQuery:
-    @pytest.mark.asyncio
     async def test_query_raises_on_empty_vector_db_ids(self):
         rag_tool = MemoryToolRuntimeImpl(config=MagicMock(), vector_io_api=MagicMock(), inference_api=MagicMock())
         with pytest.raises(ValueError):
             await rag_tool.query(content=MagicMock(), vector_db_ids=[])
 
-    @pytest.mark.asyncio
     async def test_query_chunk_metadata_handling(self):
         rag_tool = MemoryToolRuntimeImpl(config=MagicMock(), vector_io_api=MagicMock(), inference_api=MagicMock())
         content = "test query content"