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/conftest.py

@@ -8,6 +8,8 @@ depend on the registry being populated should use it explicitly or via
 ``@pytest.mark.usefixtures("web_registry_populated")``.
 """
 
+from unittest.mock import patch
+
 import pytest
 
 
@@ -48,3 +50,20 @@ def web_registry_populated():
     yield
     from agent.web_search_registry import _reset_for_tests
     _reset_for_tests()
+
+
+@pytest.fixture
+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.
+
+    Opt in at module scope with
+    ``pytestmark = pytest.mark.usefixtures("disable_lazy_stt_install")``.
+    """
+    with patch("tools.transcription_tools._try_lazy_install_stt", return_value=False):
+        yield