Make it non-optional to implement supported_languages in tts.Provider (#91347)

homeassistant/components/tts/__init__.py

@@ -1,6 +1,7 @@
 """Provide functionality for TTS."""
 from __future__ import annotations
 
+from abc import ABC, abstractmethod
 import asyncio
 from collections.abc import Mapping
 from datetime import datetime
@@ -738,7 +739,7 @@ class SpeechManager:
         return data_bytes.getvalue()
 
 
-class Provider:
+class Provider(ABC):
     """Represent a single TTS provider."""
 
     hass: HomeAssistant | None = None
@@ -750,9 +751,9 @@ class Provider:
         return None
 
     @property
-    def supported_languages(self) -> list[str] | None:
+    @abstractmethod
+    def supported_languages(self) -> list[str]:
         """Return a list of supported languages."""
-        return None
 
     @property
     def supported_options(self) -> list[str] | None: