feat(gateway): centralize audio routing + FLAC support + Telegram doc fallback (#17833)

Extracted from PR #17211 (@versun) so it can land independently of the
local_command TTS provider redesign.

- Add should_send_media_as_audio(platform, ext, is_voice) in
  gateway/platforms/base.py; single source of truth for audio routing.
- Add .flac to recognized audio extensions (MEDIA regex, weixin audio
  set, send_message audio set).
- Telegram send_voice() now falls back to send_document for formats
  Telegram's Bot API can't play natively (.wav, .flac, ...) instead of
  raising; MP3/M4A still go to sendAudio, Opus/OGG still go to sendVoice.
- Route _send_telegram() in send_message_tool through a narrower
  _TELEGRAM_SEND_AUDIO_EXTS = {.mp3, .m4a} set.
- cron.scheduler._send_media_via_adapter now delegates the audio
  decision to should_send_media_as_audio so it matches the gateway.
- Update the cron live-adapter ogg test to flag [[audio_as_voice]] so
  it still routes to sendVoice under the new Telegram-specific policy.
- Tests: unit coverage for should_send_media_as_audio across platforms,
  end-to-end MEDIA routing via _process_message_background and
  GatewayRunner._deliver_media_from_response, TelegramAdapter.send_voice
  fallback for FLAC/WAV.

Co-authored-by: Versun <me+github7604@versun.org>

gateway/platforms/telegram.py

@@ -1951,8 +1951,9 @@ class TelegramAdapter(BasePlatformAdapter):
                 return SendResult(success=False, error=self._missing_media_path_error("Audio", audio_path))
             
             with open(audio_path, "rb") as audio_file:
-                # .ogg files -> send as voice (round playable bubble)
-                if audio_path.endswith((".ogg", ".opus")):
+                ext = os.path.splitext(audio_path)[1].lower()
+                # .ogg / .opus files -> send as voice (round playable bubble)
+                if ext in (".ogg", ".opus"):
                     _voice_thread = self._metadata_thread_id(metadata)
                     msg = await self._bot.send_voice(
                         chat_id=int(chat_id),
@@ -1961,8 +1962,8 @@ class TelegramAdapter(BasePlatformAdapter):
                         reply_to_message_id=int(reply_to) if reply_to else None,
                         message_thread_id=self._message_thread_id_for_send(_voice_thread),
                     )
-                else:
-                    # .mp3 and others -> send as audio file
+                elif ext in (".mp3", ".m4a"):
+                    # Telegram's Bot API sendAudio only accepts MP3 / M4A.
                     _audio_thread = self._metadata_thread_id(metadata)
                     msg = await self._bot.send_audio(
                         chat_id=int(chat_id),
@@ -1971,6 +1972,16 @@ class TelegramAdapter(BasePlatformAdapter):
                         reply_to_message_id=int(reply_to) if reply_to else None,
                         message_thread_id=self._message_thread_id_for_send(_audio_thread),
                     )
+                else:
+                    # Formats Telegram can't play natively (.wav, .flac, ...)
+                    # — fall back to document delivery instead of raising.
+                    return await self.send_document(
+                        chat_id=chat_id,
+                        file_path=audio_path,
+                        caption=caption,
+                        reply_to=reply_to,
+                        metadata=metadata,
+                    )
             return SendResult(success=True, message_id=str(msg.message_id))
         except Exception as e:
             logger.error(