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/distribution/test_context.py

@@ -13,7 +13,6 @@ import pytest
 from llama_stack.distribution.utils.context import preserve_contexts_async_generator
 
 
-@pytest.mark.asyncio
 async def test_preserve_contexts_with_exception():
     # Create context variable
     context_var = ContextVar("exception_var", default="initial")
@@ -41,7 +40,6 @@ async def test_preserve_contexts_with_exception():
     context_var.reset(token)
 
 
-@pytest.mark.asyncio
 async def test_preserve_contexts_empty_generator():
     # Create context variable
     context_var = ContextVar("empty_var", default="initial")
@@ -66,7 +64,6 @@ async def test_preserve_contexts_empty_generator():
     context_var.reset(token)
 
 
-@pytest.mark.asyncio
 async def test_preserve_contexts_across_event_loops():
     """
     Test that context variables are preserved across event loop boundaries with nested generators.