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>

tools/send_message_tool.py

@@ -40,8 +40,12 @@ _PHONE_PLATFORMS = frozenset({"signal", "sms", "whatsapp"})
 _E164_TARGET_RE = re.compile(r"^\s*\+(\d{7,15})\s*$")
 _IMAGE_EXTS = {".jpg", ".jpeg", ".png", ".webp", ".gif"}
 _VIDEO_EXTS = {".mp4", ".mov", ".avi", ".mkv", ".3gp"}
-_AUDIO_EXTS = {".ogg", ".opus", ".mp3", ".wav", ".m4a"}
+_AUDIO_EXTS = {".ogg", ".opus", ".mp3", ".wav", ".m4a", ".flac"}
 _VOICE_EXTS = {".ogg", ".opus"}
+# Telegram's Bot API sendAudio only accepts MP3 / M4A. Other audio
+# formats either route through sendVoice (Opus/OGG) or fall back to
+# document delivery.
+_TELEGRAM_SEND_AUDIO_EXTS = {".mp3", ".m4a"}
 _URL_SECRET_QUERY_RE = re.compile(
     r"([?&](?:access_token|api[_-]?key|auth[_-]?token|token|signature|sig)=)([^&#\s]+)",
     re.IGNORECASE,
@@ -740,7 +744,7 @@ async def _send_telegram(token, chat_id, message, media_files=None, thread_id=No
                         last_msg = await bot.send_voice(
                             chat_id=int_chat_id, voice=f, **thread_kwargs
                         )
-                    elif ext in _AUDIO_EXTS:
+                    elif ext in _TELEGRAM_SEND_AUDIO_EXTS:
                         last_msg = await bot.send_audio(
                             chat_id=int_chat_id, audio=f, **thread_kwargs
                         )