Bug: Session crashes when sending image to non-vision model

Description

When sending an image to a backend model that does not support vision (e.g., a custom model via a custom API endpoint configured via ANTHROPIC_BASE_URL), the entire Claude Code session becomes unresponsive. Subsequent text-only messages also fail until the session is restarted.

Steps to Reproduce

1. Configure Claude Code to use a non-vision model via custom API endpoint:

``json
{
"env": {
"ANTHROPIC_BASE_URL": "https://custom-api.example.com/anthropic",
"ANTHROPIC_MODEL": "non-vision-model"
}
}
``

1. Send an image to the model (paste or attach an image file)
2. The request fails (expected - model does not support images)
3. Bug: The entire session becomes unresponsive
4. Send a text-only message - it also fails

Expected Behavior

• Image request should fail gracefully with a user-friendly error message
• The session should remain functional for subsequent text-only requests
• User should be informed that the current model does not support image inputs

Actual Behavior

• Session crashes completely
• All subsequent requests (including text-only) fail
• User must restart Claude Code to recover

Environment

• Claude Code version: 2.1.150
• OS: Windows 10
• Backend: Custom model via third-party API endpoint

Impact

This is a poor user experience, especially for users using custom/non-Anthropic model endpoints. The error handling should gracefully degrade when a model does not support certain input types.

Suggested Fix

1. Catch image-related API errors (e.g., 400/422 responses indicating unsupported input)
2. Display a clear error message to the user
3. Keep the session alive for subsequent requests
4. Optionally: detect model capabilities on first use and warn users about unsupported features