Bug Description

Gateway response post-processing treats every MEDIA:<path> token as a real attachment directive, even when the token appears inside fenced code blocks, inline code, grep output, or other examples. It also attempts to send non-existent paths, producing noisy Failed to send media: File not found warnings.

Steps to Reproduce

1. Send a gateway response that contains a code snippet or test output with a literal MEDIA:/tmp/missing.png example.
2. Let the platform adapter post-process the response.
3. Inspect errors.log / gateway logs.

Expected Behavior

• Literal MEDIA: examples inside code should remain text.
• Non-existent MEDIA: paths should not be attempted as attachments.
• Real existing local files referenced by MEDIA: should still be delivered and removed from text.

Actual Behavior

The adapter attempts delivery for example/missing paths and logs warnings such as:

[Feishu] Failed to send media (.png): Image file not found: /tmp/img.png

Impact

This creates noisy warning/error logs during code review, grep output, or tests that include MEDIA: examples, making real delivery failures harder to spot.

Proposed Fix

Update BasePlatformAdapter.extract_media() to:

• ignore matches inside fenced code blocks and inline code,
• validate that expanded local paths exist before returning them for delivery,
• remove only accepted delivery tags from the outgoing text.