fix(gateway): tighten MEDIA extraction regex + silent skip on file-not-found

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.

gateway/platforms/base.py

@@ -2137,7 +2137,7 @@ class BasePlatformAdapter(ABC):
         # Extract MEDIA:<path> tags, allowing optional whitespace after the colon
         # and quoted/backticked paths for LLM-formatted outputs.
         media_pattern = re.compile(
-            r'''[`"']?MEDIA:\s*(?P<path>`[^`\n]+`|"[^"\n]+"|'[^'\n]+'|(?:~/|/)\S+(?:[^\S\n]+\S+)*?\.(?:png|jpe?g|gif|webp|mp4|mov|avi|mkv|webm|ogg|opus|mp3|wav|m4a|flac|epub|pdf|zip|rar|7z|docx?|xlsx?|pptx?|txt|csv|apk|ipa)(?=[\s`"',;:)\]}]|$)|\S+)[`"']?'''
+            r'''[`"']?MEDIA:\s*(?P<path>`[^`\n]+`|"[^"\n]+"|'[^'\n]+'|(?:~/|/)\S+(?:[^\S\n]+\S+)*?\.(?:png|jpe?g|gif|webp|mp4|mov|avi|mkv|webm|ogg|opus|mp3|wav|m4a|flac|epub|pdf|zip|rar|7z|docx?|xlsx?|pptx?|txt|csv|apk|ipa)(?=[\s`"',;:)\]}]|$))[`"']?'''
         )
         for match in media_pattern.finditer(content):
             path = match.group("path").strip()

gateway/platforms/mattermost.py

@@ -471,9 +471,10 @@ class MattermostAdapter(BasePlatformAdapter):
 
         p = Path(file_path)
         if not p.exists():
-            return await self.send(
-                chat_id, f"{caption or ''}\n(file not found: {file_path})", reply_to
+            logger.warning(
+                "Mattermost: local file not found, skipping: %s", file_path
             )
+            return SendResult(success=True, message_id=None)
 
         fname = file_name or p.name
         ct = mimetypes.guess_type(fname)[0] or "application/octet-stream"