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/integration/inspect/test_inspect.py

@@ -4,20 +4,17 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
-import pytest
 from llama_stack_client import LlamaStackClient
 
 from llama_stack import LlamaStackAsLibraryClient
 
 
 class TestInspect:
-    @pytest.mark.asyncio
     def test_health(self, llama_stack_client: LlamaStackAsLibraryClient | LlamaStackClient):
         health = llama_stack_client.inspect.health()
         assert health is not None
         assert health.status == "OK"
 
-    @pytest.mark.asyncio
     def test_version(self, llama_stack_client: LlamaStackAsLibraryClient | LlamaStackClient):
         version = llama_stack_client.inspect.version()
         assert version is not None