IMG-013: Missing Content-Length Validation

[murdore]

Summary

URL downloader accepts responses with content-length: 0, resulting in empty downloads.

Root Cause

Lines 551-554 don't check if content-length is 0.

Fix

Add check if len === 0 and throw error before downloading.

Acceptance Criteria

• [ ] Check if content-length header exists and is > 0
• [ ] Throw error if content-length is 0
• [ ] Add test for 0-length response rejection