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/fixtures.py

@@ -4,14 +4,14 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
-import pytest_asyncio
+import pytest
 
 from llama_stack.distribution.store.registry import CachedDiskDistributionRegistry, DiskDistributionRegistry
 from llama_stack.providers.utils.kvstore.config import SqliteKVStoreConfig
 from llama_stack.providers.utils.kvstore.sqlite import SqliteKVStoreImpl
 
 
-@pytest_asyncio.fixture(scope="function")
+@pytest.fixture(scope="function")
 async def sqlite_kvstore(tmp_path):
     db_path = tmp_path / "test_kv.db"
     kvstore_config = SqliteKVStoreConfig(db_path=db_path.as_posix())
@@ -20,14 +20,14 @@ async def sqlite_kvstore(tmp_path):
     yield kvstore
 
 
-@pytest_asyncio.fixture(scope="function")
+@pytest.fixture(scope="function")
 async def disk_dist_registry(sqlite_kvstore):
     registry = DiskDistributionRegistry(sqlite_kvstore)
     await registry.initialize()
     yield registry
 
 
-@pytest_asyncio.fixture(scope="function")
+@pytest.fixture(scope="function")
 async def cached_disk_dist_registry(sqlite_kvstore):
     registry = CachedDiskDistributionRegistry(sqlite_kvstore)
     await registry.initialize()