refactor(tests): one runner everywhere — xdist removed, per-file isolation on every host

pytest-xdist is gone: run_tests.sh no longer dispatches on host, the
Windows xdist (--dist loadfile) arm is deleted, and pytest-xdist is
dropped from the dev extra (uv lock removes it + execnet). The
per-file subprocess runner (run_tests_parallel.py) is THE runner on
every host — the shape the CI linux lane already used.

- tests-os.yml: the macOS/Windows lanes now run scripts/run_tests.sh
  like every other lane, passing the marker-narrowed file list via
  --files and -m after --. The runner natively tolerates per-file
  empty collections (exit 5, platform-gated file) and fails the run
  when NOTHING collected — replacing the hand-rolled exit-5 branch.
- tests/gateway/conftest.py: the hasattr(config, 'workerinput')
  controller-guard was xdist-only dead code; the file-locked cache
  already handled the per-file model. Removed.
- tests/tools/test_browser_supervisor.py: the port came from xdist's
  worker_id (fixed 9225 under per-file isolation — concurrent files
  collide). Binds an ephemeral port instead (bind 0, read back).
- ~35 comment sites named xdist as the isolation mechanism; they now
  describe the shared-process hazard they actually guard against
  (bare pytest runs, same-file ordering) without naming a runner that
  no longer exists.

tests/providers/test_e2e_wiring.py

@@ -1,7 +1,8 @@
 """E2E tests: verify _build_kwargs_from_profile produces correct output.
 
 These tests call _build_kwargs_from_profile on the transport directly,
-without importing run_agent (which would cause xdist worker contamination).
+without importing run_agent (which would contaminate other tests' imports
+via shared module state).
 """
 
 import pytest