Address teknium1 review on PR #47125: the existing adapter-level tests cover
the prerequisites (_download_voice / _collect_media / _extract_text) but not
the gateway-runner handoff. Add TestWeixinVoiceGatewayHandoff covering the
final routing contract:
- An inbound Weixin voice item carrying Tencent Cloud text is surfaced as a
VOICE MessageEvent whose media is audio/silk (the shape the runner keys off
to enter Hermes' STT pipeline), exercised through the real _process_message.
- That VOICE event's body does NOT leak Tencent's STT text, so the central
transcript replaces it rather than being trusted.
- A VOICE/audio/silk event reaches the real GatewayRunner
_enrich_message_with_transcription (patched as a spy), proving the runner
handoff the adapter-only tests missed.
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