test: convert the last host-OS fakes and guard double markers

Six test files still selected an OS branch with a faked host. Each one now
carries the marker for the host that owns the branch, or derives the
expectation from the real host:

- test_clipboard: macos_only on the has_clipboard_image dispatch. The fake
  picked the branch, but _macos_has_image needs osascript.
- test_claw: windows_only on the tasklist/powershell scan, with return_value
  in place of a side_effect list that pinned the call count.
- test_linux_desktop_entry: the parametrize over "darwin"/"win32" becomes one
  marked test per host. A fake left POSIX paths and a POSIX XDG layout.
- test_graphical_browser_detection: linux_only on the display-server arm. The
  $BROWSER check runs before the platform branch, so its test stays unmarked.
- test_auth_nous_provider: the fixture pinned linux so the macOS certifi
  fallback could not change the result. The assertion now reads the host, so
  the macOS lane covers the fallback too.
- test_tts_macos_output and test_voice_mode: the afplay policy exists because
  CoreAudio init raises a TCC prompt, which no Linux runner reproduces.

tests/conftest.py refuses collection when one test carries two OS markers.
Each marker skips on all but one host, so two of them make a test that runs
nowhere while every lane reports green. tests/test_os_marker_gating.py pins
that behavior.

The docstring on TestConfirmDestructiveSlash said the Windows job runs it.
The class has no marker, so -m windows_only deselects it.

tests/test_os_marker_gating.py

@@ -0,0 +1,60 @@
+"""The collection guard against a test carrying two host-OS markers.
+
+Every marker in ``_OS_MARKS`` skips on all but one host, so two of them on one
+item means it runs on no host at all while both the Linux suite and the
+tests-os lanes report green. tests/conftest.py fails collection instead; this
+pins that behaviour so the guard can't be dropped silently.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from tests.conftest import _OS_MARKS, _reject_multiple_os_marks
+
+
+class _FakeItem:
+    """Stands in for a collected item: the guard reads only these two."""
+
+    def __init__(self, nodeid: str, *marks: str) -> None:
+        self.nodeid = nodeid
+        self._marks = [getattr(pytest.mark, name).mark for name in marks]
+
+    def iter_markers(self):
+        return iter(self._marks)
+
+
+def test_single_os_marker_is_accepted():
+    items = [_FakeItem(f"t.py::test_{name}", name) for name in _OS_MARKS]
+    _reject_multiple_os_marks(items)  # must not raise
+
+
+def test_unmarked_and_non_os_markers_are_accepted():
+    _reject_multiple_os_marks([
+        _FakeItem("t.py::test_plain"),
+        _FakeItem("t.py::test_other", "slow", "integration"),
+    ])
+
+
+def test_two_os_markers_fail_collection():
+    items = [
+        _FakeItem("t.py::test_ok", "linux_only"),
+        _FakeItem("t.py::test_bad", "linux_only", "windows_only"),
+    ]
+    with pytest.raises(pytest.UsageError) as excinfo:
+        _reject_multiple_os_marks(items)
+
+    message = str(excinfo.value)
+    assert "t.py::test_bad" in message
+    assert "linux_only, windows_only" in message
+    # The passing item must not be named — the error is a list of offenders.
+    assert "t.py::test_ok" not in message
+
+
+def test_all_three_markers_are_reported_together():
+    item = _FakeItem("t.py::test_worst", *_OS_MARKS)
+    with pytest.raises(pytest.UsageError) as excinfo:
+        _reject_multiple_os_marks([item])
+
+    for name in _OS_MARKS:
+        assert name in str(excinfo.value)