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/providers/vector_io/test_faiss.py

@@ -9,7 +9,6 @@ from unittest.mock import AsyncMock, MagicMock, patch
 
 import numpy as np
 import pytest
-import pytest_asyncio
 
 from llama_stack.apis.files import Files
 from llama_stack.apis.inference import EmbeddingsResponse, Inference
@@ -91,13 +90,13 @@ def faiss_config():
     return config
 
 
-@pytest_asyncio.fixture
+@pytest.fixture
 async def faiss_index(embedding_dimension):
     index = await FaissIndex.create(dimension=embedding_dimension)
     yield index
 
 
-@pytest_asyncio.fixture
+@pytest.fixture
 async def faiss_adapter(faiss_config, mock_inference_api, mock_files_api) -> FaissVectorIOAdapter:
     # Create the adapter
     adapter = FaissVectorIOAdapter(config=faiss_config, inference_api=mock_inference_api, files_api=mock_files_api)
@@ -113,7 +112,6 @@ async def faiss_adapter(faiss_config, mock_inference_api, mock_files_api) -> Fai
         yield adapter
 
 
-@pytest.mark.asyncio
 async def test_faiss_query_vector_returns_infinity_when_query_and_embedding_are_identical(
     faiss_index, sample_chunks, sample_embeddings, embedding_dimension
 ):
@@ -136,7 +134,6 @@ async def test_faiss_query_vector_returns_infinity_when_query_and_embedding_are_
         assert response.chunks[1] == sample_chunks[1]
 
 
-@pytest.mark.asyncio
 async def test_health_success():
     """Test that the health check returns OK status when faiss is working correctly."""
     # Create a fresh instance of FaissVectorIOAdapter for testing
@@ -160,7 +157,6 @@ async def test_health_success():
         mock_index_flat.assert_called_once_with(128)  # VECTOR_DIMENSION is 128
 
 
-@pytest.mark.asyncio
 async def test_health_failure():
     """Test that the health check returns ERROR status when faiss encounters an error."""
     # Create a fresh instance of FaissVectorIOAdapter for testing