When streaming is enabled, the GatewayStreamConsumer sends raw text
chunks directly to the platform without post-processing. This causes
MEDIA:/path/to/file tags and [[audio_as_voice]] directives to appear
as visible text in the user's chat instead of being stripped.
The non-streaming path already handles this correctly via
extract_media() in base.py, but the streaming path was missing
equivalent cleanup.
Add _clean_for_display() to GatewayStreamConsumer that strips MEDIA:
tags and internal markers before any text reaches the platform. The
actual media file delivery is unaffected — _deliver_media_from_response()
in gateway/run.py still extracts files from the agent's final_response
(separate from the stream consumer's display text).
Reported by Ao [FotM] on Discord.
