[BUG] Read tool on malformed image file causes unrecoverable session crash

Hi — thanks for the great tool. Ran into a reproducible crash scenario and traced the root cause; hope this helps.

What's Wrong?

Reading a file with an image extension (.png, .jpg) that contains invalid binary data crashes the session permanently. The error persists across --resume and /clear because the malformed payload is stored in conversation history.

Steps to Reproduce

Warning: This will crash the active session and --resume will not recover it. Test in a disposable session or use a subagent (Task tool) to isolate the impact.

python3 -c "
import os
with open('/tmp/corrupt.png', 'wb') as f:
    f.write(b'\x89PNG\r\n\x1a\n' + os.urandom(2048))
"

Then in Claude Code: Read /tmp/corrupt.png. Session becomes unrecoverable.

Root Cause

1. Read tool uses file extension to decide image handling → .png triggers base64 path
2. Contents are base64-encoded and sent to API without validating image integrity
3. Garbage bytes are interpreted as dimensions exceeding the 8000px API limit → 400 error
4. Malformed payload is persisted in conversation history → --resume replays the same error
5. No recovery path: session is permanently broken

What Should Happen?

At minimum, fall back to text-mode (hex dump or error message) when image validation fails, rather than sending unvalidated content to the API and persisting it.

Related Issues

May share a root cause with #13480, #12351, #28485, #2939.