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_sse.py

@@ -7,13 +7,10 @@
 import asyncio
 from unittest.mock import AsyncMock, MagicMock
 
-import pytest
-
 from llama_stack.apis.common.responses import PaginatedResponse
 from llama_stack.distribution.server.server import create_dynamic_typed_route, create_sse_event, sse_generator
 
 
-@pytest.mark.asyncio
 async def test_sse_generator_basic():
     # An AsyncIterator wrapped in an Awaitable, just like our web methods
     async def async_event_gen():
@@ -35,7 +32,6 @@ async def test_sse_generator_basic():
     assert seen_events[1] == create_sse_event("Test event 2")
 
 
-@pytest.mark.asyncio
 async def test_sse_generator_client_disconnected():
     # An AsyncIterator wrapped in an Awaitable, just like our web methods
     async def async_event_gen():
@@ -58,7 +54,6 @@ async def test_sse_generator_client_disconnected():
     assert seen_events[0] == create_sse_event("Test event 1")
 
 
-@pytest.mark.asyncio
 async def test_sse_generator_client_disconnected_before_response_starts():
     # Disconnect before the response starts
     async def async_event_gen():
@@ -75,7 +70,6 @@ async def test_sse_generator_client_disconnected_before_response_starts():
     assert len(seen_events) == 0
 
 
-@pytest.mark.asyncio
 async def test_sse_generator_error_before_response_starts():
     # Raise an error before the response starts
     async def async_event_gen():
@@ -93,7 +87,6 @@ async def test_sse_generator_error_before_response_starts():
     assert 'data: {"error":' in seen_events[0]
 
 
-@pytest.mark.asyncio
 async def test_paginated_response_url_setting():
     """Test that PaginatedResponse gets url set to route path."""