## What changed
After resolving startup configuration and model defaults, reject `features.token_budget.use_history_notes_extension` when the starting model lacks `supports_experimental_context`. Return an error directing users to disable the option or select a compatible model.
## Testing
Add startup coverage for explicit configuration and model defaults, verifying rejection for unsupported models and successful activation for supported models and standalone token budgets. Update history-notes test fixtures to declare experimental context support.
GitOrigin-RevId: abf1a024efc5acf97cfc858fbb93821363769dc0
## What changed
- Convert history backend `images` into `input_image` function-call output items alongside encrypted or plaintext history content.
- Keep image data out of logged output and post-tool-use hook responses.
- Reject malformed image attachments instead of silently dropping them.
## Testing
- Cover encrypted and plaintext outputs, supported image detail values, malformed attachments, and propagation into the next model request.
GitOrigin-RevId: a8eee10eb09c637bcfb06a759dd6313caf707e51
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