Preserve image detail in app-server inputs (#20693)

## Summary

- Add optional image detail to user image inputs across core, app-server
v2, thread history/event mapping, and the generated app-server
schemas/types.
- Preserve requested detail when serializing Responses image inputs:
omitted detail stays on the existing `high` default, while explicit
`original` keeps local images on the original-resolution path.
- Support `high`/`original` consistently for tool image outputs,
including MCP `codex/imageDetail`, code-mode image helpers, and
`view_image`.

codex-rs/rollout/src/tests.rs

@@ -1404,6 +1404,7 @@ async fn test_updated_at_uses_file_mtime() -> Result<()> {
             images: None,
             text_elements: Vec::new(),
             local_images: Vec::new(),
+            ..Default::default()
         })),
     };
     writeln!(file, "{}", serde_json::to_string(&user_event_line)?)?;