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/utils/memory/test_vector_store.py

@@ -13,7 +13,6 @@ from llama_stack.apis.tools import RAGDocument
 from llama_stack.providers.utils.memory.vector_store import content_from_data_and_mime_type, content_from_doc
 
 
-@pytest.mark.asyncio
 async def test_content_from_doc_with_url():
     """Test extracting content from RAGDocument with URL content."""
     mock_url = URL(uri="https://example.com")
@@ -33,7 +32,6 @@ async def test_content_from_doc_with_url():
         mock_instance.get.assert_called_once_with(mock_url.uri)
 
 
-@pytest.mark.asyncio
 async def test_content_from_doc_with_pdf_url():
     """Test extracting content from RAGDocument with URL pointing to a PDF."""
     mock_url = URL(uri="https://example.com/document.pdf")
@@ -58,7 +56,6 @@ async def test_content_from_doc_with_pdf_url():
         mock_parse_pdf.assert_called_once_with(b"PDF binary data")
 
 
-@pytest.mark.asyncio
 async def test_content_from_doc_with_data_url():
     """Test extracting content from RAGDocument with data URL content."""
     data_url = "data:text/plain;base64,SGVsbG8gV29ybGQ="  # "Hello World" base64 encoded
@@ -74,7 +71,6 @@ async def test_content_from_doc_with_data_url():
         mock_content_from_data.assert_called_once_with(data_url)
 
 
-@pytest.mark.asyncio
 async def test_content_from_doc_with_string():
     """Test extracting content from RAGDocument with string content."""
     content_string = "This is plain text content"
@@ -85,7 +81,6 @@ async def test_content_from_doc_with_string():
     assert result == content_string
 
 
-@pytest.mark.asyncio
 async def test_content_from_doc_with_string_url():
     """Test extracting content from RAGDocument with string URL content."""
     url_string = "https://example.com"
@@ -105,7 +100,6 @@ async def test_content_from_doc_with_string_url():
         mock_instance.get.assert_called_once_with(url_string)
 
 
-@pytest.mark.asyncio
 async def test_content_from_doc_with_string_pdf_url():
     """Test extracting content from RAGDocument with string URL pointing to a PDF."""
     url_string = "https://example.com/document.pdf"
@@ -130,7 +124,6 @@ async def test_content_from_doc_with_string_pdf_url():
         mock_parse_pdf.assert_called_once_with(b"PDF binary data")
 
 
-@pytest.mark.asyncio
 async def test_content_from_doc_with_interleaved_content():
     """Test extracting content from RAGDocument with InterleavedContent (the new case added in the commit)."""
     interleaved_content = [TextContentItem(text="First item"), TextContentItem(text="Second item")]