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`

pyproject.toml

@@ -58,9 +58,9 @@ ui = [
 
 [dependency-groups]
 dev = [
-    "pytest",
+    "pytest>=8.4",
     "pytest-timeout",
-    "pytest-asyncio",
+    "pytest-asyncio>=1.0",
     "pytest-cov",
     "pytest-html",
     "pytest-json-report",
@@ -339,3 +339,6 @@ warn_required_dynamic_aliases = true
 
 [tool.ruff.lint.pep8-naming]
 classmethod-decorators = ["classmethod", "pydantic.field_validator"]
+
+[tool.pytest.ini_options]
+asyncio_mode = "auto"