[Bugfix] Replace hardcoded Vivian speaker default with config-ordered default (#5814)

Signed-off-by: khairulkabir1661 <khairulkabir1661@users.noreply.github.com>
Co-authored-by: khairulkabir1661 <khairulkabir1661@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Alex Brooks <albrooks@redhat.com>

tests/entrypoints/openai_api/test_serving_speech.py

@@ -2535,7 +2535,7 @@ class TestTTSMethods:
         )
 
         assert speech_server._is_tts is False
-        assert load_supported_speakers(speech_server.engine_client) == set()
+        assert load_supported_speakers(speech_server.engine_client) == []
         warning.assert_not_called()
 
     def test_load_supported_languages_from_config(self, speech_server):