review(stt-xai): address cetej's nits

- Replace hardcoded 'fr' default with DEFAULT_LOCAL_STT_LANGUAGE ('en')
  — removes locale leak, matches other providers
- Drop redundant default=True on is_truthy_value (dict .get already defaults)
- Update auto-detect comment to include 'xai' in the chain
- Fix docstring: 21 languages (match PR body + actual xAI API)
- Update test_sends_language_and_format to set HERMES_LOCAL_STT_LANGUAGE=fr
  explicitly, since default is no longer 'fr'

All 18 xAI STT tests pass locally.

tests/tools/test_transcription_tools.py

@@ -1109,6 +1109,9 @@ class TestTranscribeXAI:
 
     def test_sends_language_and_format(self, monkeypatch, sample_ogg, mock_xai_http_module):
         monkeypatch.setenv("XAI_API_KEY", "xai-test-key")
+        # Explicitly set language via env to exercise the override chain
+        # (config > env > DEFAULT_LOCAL_STT_LANGUAGE)
+        monkeypatch.setenv("HERMES_LOCAL_STT_LANGUAGE", "fr")
 
         mock_response = MagicMock()
         mock_response.status_code = 200