Support original-detail metadata on MCP image outputs (#17714)

## Summary
- honor `_meta["codex/imageDetail"] == "original"` on MCP image content
and map it to `detail: "original"` where supported
- strip that detail back out when the active model does not support
original-detail image inputs
- update code-mode `image(...)` to accept individual MCP image blocks
- teach `js_repl` / `codex.emitImage(...)` to preserve the same hint
from raw MCP image outputs
- document the new `_meta` contract and add generic RMCP-backed coverage
across protocol, core, code-mode, and js_repl paths

codex-rs/code-mode/src/runtime/callbacks.rs

@@ -109,7 +109,20 @@ pub(super) fn image_callback(
     } else {
         args.get(0)
     };
-    let image_item = match normalize_output_image(scope, value) {
+    let detail_override = if args.length() < 2 {
+        None
+    } else {
+        let detail = args.get(1);
+        if detail.is_string() {
+            Some(detail.to_rust_string_lossy(scope))
+        } else if detail.is_null() || detail.is_undefined() {
+            None
+        } else {
+            throw_type_error(scope, "image detail must be a string when provided");
+            return;
+        }
+    };
+    let image_item = match normalize_output_image(scope, value, detail_override) {
         Ok(image_item) => image_item,
         Err(()) => return,
     };