Summary

• Ignore MEDIA: tags inside fenced code blocks and inline code.
• Only extract MEDIA: paths for native delivery when the expanded local file exists.
• Preserve skipped MEDIA: examples in text instead of removing them or attempting delivery.

Root cause

BasePlatformAdapter.extract_media() treated any MEDIA:<path> token as a real attachment directive. Code snippets, grep output, and test examples containing MEDIA:/tmp/missing.png were therefore sent through native media delivery and produced noisy File not found warnings.

What changed

• Builds code spans for fenced and inline code and skips matches inside them.
• Validates candidate paths with os.path.isfile() before adding them to the media list.
• Removes only accepted delivery tags from the outgoing text; skipped examples remain visible.
• Updates gateway media extraction tests to use real temp files for positive cases and adds coverage for missing files / code examples.

Tests

• python -m pytest tests/gateway/test_media_extraction.py tests/gateway/test_platform_base.py -q -o 'addopts='

Verification

• Verified that existing local media files are still extracted and removed from text.
• Verified that missing paths, fenced-code examples, and inline-code examples are not extracted and are preserved.

Fixes #16434