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/run.py

@@ -7179,14 +7179,15 @@ class GatewayRunner:
 
             _thread_meta = {"thread_id": event.source.thread_id} if event.source.thread_id else None
 
-            _AUDIO_EXTS = {'.ogg', '.opus', '.mp3', '.wav', '.m4a'}
+            from gateway.platforms.base import should_send_media_as_audio
+
             _VIDEO_EXTS = {'.mp4', '.mov', '.avi', '.mkv', '.webm', '.3gp'}
             _IMAGE_EXTS = {'.jpg', '.jpeg', '.png', '.webp', '.gif'}
 
             for media_path, is_voice in media_files:
                 try:
                     ext = Path(media_path).suffix.lower()
-                    if ext in _AUDIO_EXTS:
+                    if should_send_media_as_audio(event.source.platform, ext, is_voice=is_voice):
                         await adapter.send_voice(
                             chat_id=event.source.chat_id,
                             audio_path=media_path,