Support multimodal custom tool outputs (#12948)

## Summary

This changes `custom_tool_call_output` to use the same output payload
shape as `function_call_output`, so freeform tools can return either
plain text or structured content items.

The main goal is to let `js_repl` return image content from nested
`view_image` calls in its own `custom_tool_call_output`, instead of
relying on a separate injected message.

## What changed

- Changed `custom_tool_call_output.output` from `string` to
`FunctionCallOutputPayload`
- Updated freeform tool plumbing to preserve structured output bodies
- Updated `js_repl` to aggregate nested tool content items and attach
them to the outer `js_repl` result
- Removed the old `js_repl` special case that injected `view_image`
results as a separate pending user image message
- Updated normalization/history/truncation paths to handle multimodal
`custom_tool_call_output`
- Regenerated app-server protocol schema artifacts

## Behavior

Direct `view_image` calls still return a `function_call_output` with
image content.

When `view_image` is called inside `js_repl`, the outer `js_repl`
`custom_tool_call_output` now carries:
- an `input_text` item if the JS produced text output
- one or more `input_image` items from nested tool results

So the nested image result now stays inside the `js_repl` tool output
instead of being injected as a separate message.

## Compatibility

This is intended to be backward-compatible for resumed conversations.

Older histories that stored `custom_tool_call_output.output` as a plain
string still deserialize correctly, and older histories that used the
previous injected-image-message flow also continue to resume.

Added regression coverage for resuming a pre-change rollout containing:
- string-valued `custom_tool_call_output`
- legacy injected image message history


#### [git stack](https://github.com/magus/git-stack-cli)
- 👉 `1` https://github.com/openai/codex/pull/12948

codex-rs/core/src/tools/context.rs

@@ -95,15 +95,12 @@ impl ToolOutput {
         match self {
             ToolOutput::Function { body, success } => {
                 // `custom_tool_call` is the Responses API item type for freeform
-                // tools (`ToolSpec::Freeform`, e.g. freeform `apply_patch`).
-                // Those payloads must round-trip as `custom_tool_call_output`
-                // with plain string output.
+                // tools (`ToolSpec::Freeform`, e.g. freeform `apply_patch` or
+                // `js_repl`).
                 if matches!(payload, ToolPayload::Custom { .. }) {
-                    // Freeform/custom tools (`custom_tool_call`) use the custom
-                    // output wire shape and remain string-only.
                     return ResponseInputItem::CustomToolCallOutput {
                         call_id: call_id.to_string(),
-                        output: body.to_text().unwrap_or_default(),
+                        output: FunctionCallOutputPayload { body, success },
                     };
                 }
 
@@ -183,7 +180,9 @@ mod tests {
         match response {
             ResponseInputItem::CustomToolCallOutput { call_id, output } => {
                 assert_eq!(call_id, "call-42");
-                assert_eq!(output, "patched");
+                assert_eq!(output.text_content(), Some("patched"));
+                assert!(output.content_items().is_none());
+                assert_eq!(output.success, Some(true));
             }
             other => panic!("expected CustomToolCallOutput, got {other:?}"),
         }
@@ -234,8 +233,21 @@ mod tests {
 
         match response {
             ResponseInputItem::CustomToolCallOutput { call_id, output } => {
+                let expected = vec![
+                    FunctionCallOutputContentItem::InputText {
+                        text: "line 1".to_string(),
+                    },
+                    FunctionCallOutputContentItem::InputImage {
+                        image_url: "data:image/png;base64,AAA".to_string(),
+                    },
+                    FunctionCallOutputContentItem::InputText {
+                        text: "line 2".to_string(),
+                    },
+                ];
                 assert_eq!(call_id, "call-99");
-                assert_eq!(output, "line 1\nline 2");
+                assert_eq!(output.content_items(), Some(expected.as_slice()));
+                assert_eq!(output.body.to_text().as_deref(), Some("line 1\nline 2"));
+                assert_eq!(output.success, Some(true));
             }
             other => panic!("expected CustomToolCallOutput, got {other:?}"),
         }