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>

tests/cron/test_scheduler.py

@@ -551,14 +551,14 @@ class TestDeliverResultWrapping:
              patch("asyncio.run_coroutine_threadsafe", side_effect=fake_run_coro):
             _deliver_result(
                 job,
-                "MEDIA:/tmp/voice.ogg",
+                "[[audio_as_voice]]\nMEDIA:/tmp/voice.ogg",
                 adapters={Platform.TELEGRAM: adapter},
                 loop=loop,
             )
 
         # Text send should NOT be called (no text after stripping MEDIA tag)
         adapter.send.assert_not_called()
-        # Audio should still be delivered
+        # Audio should still be delivered as a voice bubble
         adapter.send_voice.assert_called_once()
 
     def test_live_adapter_sends_cleaned_text_not_raw(self):