fix: address PR review round 5 — streaming guard, VC auth, history prefix, auto-TTS control

1. Gate _streaming_api_call to chat_completions mode only — Anthropic and
   Codex fall back to _interruptible_api_call. Preserve Anthropic base_url
   across all client rebuild paths (interrupt, fallback, 401 refresh).

2. Discord VC synthetic events now use chat_type="channel" instead of
   defaulting to "dm" — prevents session bleed into DM context.
   Authorization runs before echoing transcript. Sanitize @everyone/@here
   in voice transcripts.

3. CLI voice prefix ("[Voice input...]") is now API-call-local only —
   stripped from returned history so it never persists to session DB or
   resumed sessions.

4. /voice off now disables base adapter auto-TTS via _auto_tts_disabled_chats
   set — voice input no longer triggers TTS when voice mode is off.

tests/gateway/test_voice_command.py

@@ -38,6 +38,7 @@ def _make_runner(tmp_path):
     runner._VOICE_MODE_PATH = tmp_path / "gateway_voice_mode.json"
     runner._session_db = None
     runner.session_store = MagicMock()
+    runner._is_user_authorized = lambda source: True
     return runner
 
 
@@ -731,6 +732,7 @@ class TestVoiceChannelCommands:
         assert event.text == "Hello from VC"
         assert event.message_type == MessageType.VOICE
         assert event.source.chat_id == "123"
+        assert event.source.chat_type == "channel"
 
     @pytest.mark.asyncio
     async def test_input_posts_transcript_in_text_channel(self, runner):