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/registry/test_registry_acl.py

@@ -5,14 +5,11 @@
 # the root directory of this source tree.
 
 
-import pytest
-
 from llama_stack.apis.models import ModelType
 from llama_stack.distribution.datatypes import ModelWithOwner, User
 from llama_stack.distribution.store.registry import CachedDiskDistributionRegistry
 
 
-@pytest.mark.asyncio
 async def test_registry_cache_with_acl(cached_disk_dist_registry):
     model = ModelWithOwner(
         identifier="model-acl",
@@ -48,7 +45,6 @@ async def test_registry_cache_with_acl(cached_disk_dist_registry):
     assert new_model.owner.attributes["teams"] == ["ai-team"]
 
 
-@pytest.mark.asyncio
 async def test_registry_empty_acl(cached_disk_dist_registry):
     model = ModelWithOwner(
         identifier="model-empty-acl",
@@ -85,7 +81,6 @@ async def test_registry_empty_acl(cached_disk_dist_registry):
     assert len(all_models) == 2
 
 
-@pytest.mark.asyncio
 async def test_registry_serialization(cached_disk_dist_registry):
     attributes = {
         "roles": ["admin", "researcher"],