feat(app-server, core): allow text + image content items for dynamic tool outputs (#10567)

Took over the work that @aaronl-openai started here:
https://github.com/openai/codex/pull/10397

Now that app-server clients are able to set up custom tools (called
`dynamic_tools` in app-server), we should expose a way for clients to
pass in not just text, but also image outputs. This is something the
Responses API already supports for function call outputs, where you can
pass in either a string or an array of content outputs (text, image,
file):
https://platform.openai.com/docs/api-reference/responses/create#responses_create-input-input_item_list-item-function_tool_call_output-output-array-input_image

So let's just plumb it through in Codex (with the caveat that we only
support text and image for now). This is implemented end-to-end across
app-server v2 protocol types and core tool handling.

## Breaking API change
NOTE: This introduces a breaking change with dynamic tools, but I think
it's ok since this concept was only recently introduced
(https://github.com/openai/codex/pull/9539) and it's better to get the
API contract correct. I don't think there are any real consumers of this
yet (not even the Codex App).

Old shape:
`{ "output": "dynamic-ok", "success": true }`

New shape:
```
{
    "contentItems": [
      { "type": "inputText", "text": "dynamic-ok" },
      { "type": "inputImage", "imageUrl": "data:image/png;base64,AAA" }
    ]
  "success": true
}
```

codex-rs/app-server/src/dynamic_tools.rs

@@ -1,5 +1,6 @@
 use codex_app_server_protocol::DynamicToolCallResponse;
 use codex_core::CodexThread;
+use codex_protocol::dynamic_tools::DynamicToolCallOutputContentItem as CoreDynamicToolCallOutputContentItem;
 use codex_protocol::dynamic_tools::DynamicToolResponse as CoreDynamicToolResponse;
 use codex_protocol::protocol::Op;
 use std::sync::Arc;
@@ -17,8 +18,9 @@ pub(crate) async fn on_call_response(
         Err(err) => {
             error!("request failed: {err:?}");
             let fallback = CoreDynamicToolResponse {
-                call_id: call_id.clone(),
-                output: "dynamic tool request failed".to_string(),
+                content_items: vec![CoreDynamicToolCallOutputContentItem::InputText {
+                    text: "dynamic tool request failed".to_string(),
+                }],
                 success: false,
             };
             if let Err(err) = conversation
@@ -37,14 +39,25 @@ pub(crate) async fn on_call_response(
     let response = serde_json::from_value::<DynamicToolCallResponse>(value).unwrap_or_else(|err| {
         error!("failed to deserialize DynamicToolCallResponse: {err}");
         DynamicToolCallResponse {
-            output: "dynamic tool response was invalid".to_string(),
+            content_items: vec![
+                codex_app_server_protocol::DynamicToolCallOutputContentItem::InputText {
+                    text: "dynamic tool response was invalid".to_string(),
+                },
+            ],
             success: false,
         }
     });
+
+    let DynamicToolCallResponse {
+        content_items,
+        success,
+    } = response;
     let response = CoreDynamicToolResponse {
-        call_id: call_id.clone(),
-        output: response.output,
-        success: response.success,
+        content_items: content_items
+            .into_iter()
+            .map(CoreDynamicToolCallOutputContentItem::from)
+            .collect(),
+        success,
     };
     if let Err(err) = conversation
         .submit(Op::DynamicToolResponse {