test(tools): centralize disable_lazy_stt_install fixture in conftest

Move the autouse `_disable_lazy_stt_install` fixture out of the three
transcription test files and into `tests/tools/conftest.py` as a regular
(non-autouse) fixture. Each transcription test module opts in once at
the top via `pytestmark = pytest.mark.usefixtures(...)`.

Why: addresses three Copilot inline review comments on this PR that
flagged the verbatim duplication across files. Centralizing also keeps
the patch target in a single place, so a future rename of
`_try_lazy_install_stt` only updates one location.

Why opt-in (not autouse in conftest): other `tests/tools/` files do not
patch `_HAS_FASTER_WHISPER` and have no reason to bypass the runtime
lazy-install probe; making the fixture autouse globally would silently
mask any future test that wants to exercise the real lazy-install path.

tests/tools/test_transcription_tools.py

@@ -42,6 +42,9 @@ def sample_ogg(tmp_path):
     return str(ogg_path)
 
 
+pytestmark = pytest.mark.usefixtures("disable_lazy_stt_install")
+
+
 @pytest.fixture(autouse=True)
 def clean_env(monkeypatch):
     """Ensure no real API keys leak into tests."""
@@ -53,20 +56,6 @@ def clean_env(monkeypatch):
     monkeypatch.delenv("HERMES_LOCAL_STT_LANGUAGE", raising=False)
 
 
-@pytest.fixture(autouse=True)
-def _disable_lazy_stt_install():
-    """Disarm the runtime lazy-install probe so static ``_HAS_FASTER_WHISPER``
-    patches accurately simulate 'faster-whisper not installed'.
-
-    Without this, ``_try_lazy_install_stt()`` calls
-    ``importlib.util.find_spec("faster_whisper")``, which returns truthy
-    whenever the package is installed in the dev / CI environment —
-    defeating the test's ``_HAS_FASTER_WHISPER=False`` patch.
-    """
-    with patch("tools.transcription_tools._try_lazy_install_stt", return_value=False):
-        yield
-
-
 # ============================================================================
 # _get_provider — full permutation matrix
 # ============================================================================