VSCode extension: pasting large image kills conversation with no recovery

Bug Description

When pasting an image that's too large into the VSCode extension, the conversation is killed with a "prompt is too big" error. There is no way to recover from this state.

Steps to Reproduce

1. Open Claude Code in the VSCode extension
2. Paste a large image into the conversation
3. The conversation dies with a "prompt is too big" error

Expected Behavior

• The extension should reject the image gracefully before sending (e.g., "Image too large, please resize or compress")
• OR if the error occurs server-side, the user should be able to continue the conversation by removing the offending message

Actual Behavior

• The conversation is permanently broken with "prompt is too big"
• Forking from a previous message does not recover the conversation
• The user is completely stuck with no way out

Environment

• Claude Code VSCode extension
• Linux (Debian 13, kernel 6.12.57)