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/gateway/test_platform_base.py

@@ -323,6 +323,55 @@ class TestExtractMedia:
         assert "Here" in cleaned
         assert "After" in cleaned
 
+    def test_media_tag_supports_unquoted_flac_paths_with_spaces(self):
+        content = "MEDIA:/tmp/Jane Doe/speech.flac"
+        media, cleaned = BasePlatformAdapter.extract_media(content)
+        assert media == [("/tmp/Jane Doe/speech.flac", False)]
+        assert cleaned == ""
+
+
+# ---------------------------------------------------------------------------
+# should_send_media_as_audio
+# ---------------------------------------------------------------------------
+
+class TestShouldSendMediaAsAudio:
+    """Audio-routing policy shared by gateway + scheduler + send_message."""
+
+    def test_unknown_extension_returns_false(self):
+        from gateway.platforms.base import should_send_media_as_audio
+        assert should_send_media_as_audio(None, ".png") is False
+        assert should_send_media_as_audio("telegram", ".pdf") is False
+
+    def test_non_telegram_platforms_route_all_audio(self):
+        from gateway.platforms.base import should_send_media_as_audio
+        for ext in (".mp3", ".m4a", ".wav", ".flac", ".ogg", ".opus"):
+            assert should_send_media_as_audio("discord", ext) is True
+            assert should_send_media_as_audio("slack", ext) is True
+
+    def test_telegram_mp3_and_m4a_route_to_audio(self):
+        from gateway.platforms.base import should_send_media_as_audio
+        assert should_send_media_as_audio("telegram", ".mp3") is True
+        assert should_send_media_as_audio("telegram", ".m4a") is True
+
+    def test_telegram_wav_and_flac_fall_through_to_document(self):
+        from gateway.platforms.base import should_send_media_as_audio
+        assert should_send_media_as_audio("telegram", ".wav") is False
+        assert should_send_media_as_audio("telegram", ".flac") is False
+
+    def test_telegram_ogg_opus_only_when_voice_flagged(self):
+        from gateway.platforms.base import should_send_media_as_audio
+        assert should_send_media_as_audio("telegram", ".ogg", is_voice=True) is True
+        assert should_send_media_as_audio("telegram", ".opus", is_voice=True) is True
+        assert should_send_media_as_audio("telegram", ".ogg") is False
+        assert should_send_media_as_audio("telegram", ".opus") is False
+
+    def test_accepts_platform_enum(self):
+        from gateway.config import Platform
+        from gateway.platforms.base import should_send_media_as_audio
+        assert should_send_media_as_audio(Platform.TELEGRAM, ".mp3") is True
+        assert should_send_media_as_audio(Platform.TELEGRAM, ".flac") is False
+        assert should_send_media_as_audio(Platform.DISCORD, ".flac") is True
+
 
 # ---------------------------------------------------------------------------
 # truncate_message