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/test_scheduler.py

@@ -11,7 +11,6 @@ import pytest
 from llama_stack.providers.utils.scheduler import JobStatus, Scheduler
 
 
-@pytest.mark.asyncio
 async def test_scheduler_unknown_backend():
     with pytest.raises(ValueError):
         Scheduler(backend="unknown")
@@ -26,7 +25,6 @@ async def wait_for_job_completed(sched: Scheduler, job_id: str) -> None:
     raise TimeoutError(f"Job {job_id} did not complete in time.")
 
 
-@pytest.mark.asyncio
 async def test_scheduler_naive():
     sched = Scheduler()
 
@@ -87,7 +85,6 @@ async def test_scheduler_naive():
     assert job.logs[0][0] < job.logs[1][0]
 
 
-@pytest.mark.asyncio
 async def test_scheduler_naive_handler_raises():
     sched = Scheduler()