## Why
Image preparation replaces uploaded images with file references in model history, but user message events still retain the original local paths or inline image URLs. Live and persisted display history should carry the prepared file IDs as well.
## What changed
- Prepare images before emitting user message events, and copy uploaded file IDs into the corresponding user inputs.
- Track original input positions through content expansion so failed image reads, omitted inputs, and duplicate URLs do not misassociate images.
- Preserve text spans, image detail settings, and inputs without a prepared file reference.
## Testing
Add a protocol test for image position mapping and an integration test covering file IDs in model requests, live user message events, and persisted display history, including failed images and duplicate URLs.
GitOrigin-RevId: 55e7ab97925b5db049c5c2403d90349635eee740
96aca987f7