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/agents/test_persistence.py

@@ -44,7 +44,6 @@ def common_params(inference_model):
     )
 
 
-@pytest.mark.asyncio
 @pytest.mark.skip(reason="This test needs to be migrated to api / client-sdk world")
 async def test_delete_agents_and_sessions(self, agents_stack, common_params):
     agents_impl = agents_stack.impls[Api.agents]
@@ -73,7 +72,6 @@ async def test_delete_agents_and_sessions(self, agents_stack, common_params):
     assert agent_response is None
 
 
-@pytest.mark.asyncio
 @pytest.mark.skip(reason="This test needs to be migrated to api / client-sdk world")
 async def test_get_agent_turns_and_steps(self, agents_stack, sample_messages, common_params):
     agents_impl = agents_stack.impls[Api.agents]