Three related fixes for the MEDIA:<path> extraction pipeline that
caused 'file not found' noise in platform channels:
1. run.py — tighten tool-result MEDIA regex from \S+ (any non-
whitespace) to require a path pattern with known extensions.
Prevents LLM-generated placeholder paths like
'MEDIA:/path/to/example.mp4' from being captured as real media.
2. base.py — remove the |\S+ fallback in extract_media() that
catches anything non-whitespace as a potential MEDIA path.
This was the primary cause of false positives — strings like
'' in tool output were captured as MEDIA: paths.
3. mattermost.py — replace the file-not-found error message sent
to the channel with a silent logger.warning() skip. When a
path extracted by MEDIA doesn't exist on disk, the channel
no longer gets a noisy '(file not found: ...)' message.
Impact: eliminates the persistent 'file not found' spam in
Mattermost channels caused by over-broad MEDIA regex patterns
matching non-path text in tool output.
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