[MCP] Render MCP tool call result images to the model (#5600)

It's pretty amazing we have gotten here without the ability for the
model to see image content from MCP tool calls.

This PR builds off of 4391 and fixes #4819. I would like @KKcorps to get
adequete credit here but I also want to get this fix in ASAP so I gave
him a week to update it and haven't gotten a response so I'm going to
take it across the finish line.


This test highlights how absured the current situation is. I asked the
model to read this image using the Chrome MCP
<img width="2378" height="674" alt="image"
src="https://github.com/user-attachments/assets/9ef52608-72a2-4423-9f5e-7ae36b2b56e0"
/>

After this change, it correctly outputs:
> Captured the page: image dhows a dark terminal-style UI labeled
`OpenAI Codex (v0.0.0)` with prompt `model: gpt-5-codex medium` and
working directory `/codex/codex-rs`
(and more)  

Before this change, it said:
> Took the full-page screenshot you asked for. It shows a long,
horizontally repeating pattern of stylized people in orange, light-blue,
and mustard clothing, holding hands in alternating poses against a white
background. No text or other graphics-just rows of flat illustration
stretching off to the right.

Without this change, the Figma, Playwright, Chrome, and other visual MCP
servers are pretty much entirely useless.

I tested this change with the openai respones api as well as a third
party completions api

codex-rs/core/src/chat_completions.rs

@@ -17,6 +17,7 @@ use crate::util::backoff;
 use bytes::Bytes;
 use codex_otel::otel_event_manager::OtelEventManager;
 use codex_protocol::models::ContentItem;
+use codex_protocol::models::FunctionCallOutputContentItem;
 use codex_protocol::models::ReasoningItemContent;
 use codex_protocol::models::ResponseItem;
 use eventsource_stream::Eventsource;
@@ -159,16 +160,26 @@ pub(crate) async fn stream_chat_completions(
     for (idx, item) in input.iter().enumerate() {
         match item {
             ResponseItem::Message { role, content, .. } => {
+                // Build content either as a plain string (typical for assistant text)
+                // or as an array of content items when images are present (user/tool multimodal).
                 let mut text = String::new();
+                let mut items: Vec<serde_json::Value> = Vec::new();
+                let mut saw_image = false;
+
                 for c in content {
                     match c {
                         ContentItem::InputText { text: t }
                         | ContentItem::OutputText { text: t } => {
                             text.push_str(t);
+                            items.push(json!({"type":"text","text": t}));
+                        }
+                        ContentItem::InputImage { image_url } => {
+                            saw_image = true;
+                            items.push(json!({"type":"image_url","image_url": {"url": image_url}}));
                         }
-                        _ => {}
                     }
                 }
+
                 // Skip exact-duplicate assistant messages.
                 if role == "assistant" {
                     if let Some(prev) = &last_assistant_text
@@ -179,7 +190,17 @@ pub(crate) async fn stream_chat_completions(
                     last_assistant_text = Some(text.clone());
                 }
 
-                let mut msg = json!({"role": role, "content": text});
+                // For assistant messages, always send a plain string for compatibility.
+                // For user messages, if an image is present, send an array of content items.
+                let content_value = if role == "assistant" {
+                    json!(text)
+                } else if saw_image {
+                    json!(items)
+                } else {
+                    json!(text)
+                };
+
+                let mut msg = json!({"role": role, "content": content_value});
                 if role == "assistant"
                     && let Some(reasoning) = reasoning_by_anchor_index.get(&idx)
                     && let Some(obj) = msg.as_object_mut()
@@ -238,10 +259,29 @@ pub(crate) async fn stream_chat_completions(
                 messages.push(msg);
             }
             ResponseItem::FunctionCallOutput { call_id, output } => {
+                // Prefer structured content items when available (e.g., images)
+                // otherwise fall back to the legacy plain-string content.
+                let content_value = if let Some(items) = &output.content_items {
+                    let mapped: Vec<serde_json::Value> = items
+                        .iter()
+                        .map(|it| match it {
+                            FunctionCallOutputContentItem::InputText { text } => {
+                                json!({"type":"text","text": text})
+                            }
+                            FunctionCallOutputContentItem::InputImage { image_url } => {
+                                json!({"type":"image_url","image_url": {"url": image_url}})
+                            }
+                        })
+                        .collect();
+                    json!(mapped)
+                } else {
+                    json!(output.content)
+                };
+
                 messages.push(json!({
                     "role": "tool",
                     "tool_call_id": call_id,
-                    "content": output.content,
+                    "content": content_value,
                 }));
             }
             ResponseItem::CustomToolCall {