test: declare pytest-asyncio so the async attach tests actually run

[FritzHeider]

Problem

tests/test_browser_attach.py has six @pytest.mark.asyncio tests, but pytest-asyncio is not declared in the dev extra. After a clean pip install -e '.[dev]', all six fail:

FAILED tests/test_browser_attach.py::test_resolve_ws_url_attach_uses_devtools_active_port
  - Failed: async def functions are not natively supported.
... (6 total)
PytestUnknownMarkWarning: Unknown pytest.mark.asyncio - is this a typo?

So the entire attach-mode test surface — WS URL resolution, port polling, and the guarantee that attach mode never kills the user's browser — is not currently being exercised.

What declaring it revealed

With pytest-asyncio installed, five of the six pass and one genuinely fails:

FAILED tests/test_browser_attach.py::test_close_browser_attach_does_not_terminate_process
  - AttributeError: 'NoneType' object has no attribute 'terminate'

close_browser() clears self._process, so reading browser._process.terminate after the call dereferences None. The assertion could never have run. Capturing the mock in a local before the call fixes it, and I added kill.assert_not_called() alongside — attach mode should leave the user's browser alone by either path.

Changes

• pyproject.toml: add pytest-asyncio>=1.0.0 to the dev extra, and set asyncio_default_fixture_loop_scope = "function" to silence the unset-default deprecation warning.
• uv.lock: regenerated for the new dependency.
• tests/test_browser_attach.py: capture the process mock before close_browser(), and assert kill() wasn't called either.

Result

pytest tests/ goes from 1 failed, 19 passed to 20 passed.

Branches off current main; no other files touched.