Add isolated test where ensure('stt.mistral') raises FeatureUnavailable
but the raw mistralai.client.Mistral import succeeds, verifying the
transcription_tools.py fallthrough path introduced in the same PR.
Replace
aise ImportError(str(e)) with pass in the except Exception
handler of _import_edge_tts(), _import_elevenlabs(), and
_import_mistral_client() so packages installed via PYTHONPATH or Docker
layered filesystems still work when lazy_deps.ensure() raises.
Also fix the Mistral STT path in transcription_tools.py which only
caught ImportError, not FeatureUnavailable.
Adds 6 regression tests using sys.modules fixtures (no
builtins.__import__ patching).
