Add code_mode output helpers for text and images (#14244)

Summary
- document how code-mode can import `output_text`/`output_image` and
ensure `add_content` stays compatible
- add a synthetic `@openai/code_mode` module that appends content items
and validates inputs
- cover the new behavior with integration tests for structured text and
image outputs

Testing
- Not run (not requested)

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

@@ -1620,7 +1620,7 @@ source: /[\s\S]+/
         enabled_tool_names.join(", ")
     };
     let description = format!(
-        "Runs JavaScript in a Node-backed `node:vm` context. This is a freeform tool: send raw JavaScript source text (no JSON/quotes/markdown fences). Direct tool calls remain available while `code_mode` is enabled. Inside JavaScript, import nested tools from `tools.js`, for example `import {{ exec_command }} from \"tools.js\"` or `import {{ tools }} from \"tools.js\"`. `tools[name]` and identifier wrappers like `await shell(args)` remain available for compatibility when the tool name is a valid JS identifier. Nested tool calls resolve to their code-mode result values. Import `set_max_output_tokens_per_exec_call` from `@openai/code_mode` to set the token budget used to truncate the final Rust-side result of the current `code_mode` execution; the default is `10000`. This guards the overall `code_mode` output, not individual nested tool invocations. When truncation happens, the final text uses the unified-exec style `Original token count:` / `Output:` wrapper and the usual `…N tokens truncated…` marker. Function tools require JSON object arguments. Freeform tools require raw strings. Use synchronous `add_content(value)` with a content item, content-item array, or string. Structured nested-tool results should be converted to text first, for example with `JSON.stringify(...)`. Only content passed to `add_content(value)` is surfaced back to the model. Enabled nested tools: {enabled_list}."
+        "Runs JavaScript in a Node-backed `node:vm` context. This is a freeform tool: send raw JavaScript source text (no JSON/quotes/markdown fences). Direct tool calls remain available while `code_mode` is enabled. Inside JavaScript, import nested tools from `tools.js`, for example `import {{ exec_command }} from \"tools.js\"` or `import {{ tools }} from \"tools.js\"`. Namespaced tools are also available from `tools/<namespace...>.js`; MCP tools use `tools/mcp/<server>.js`, for example `import {{ append_notebook_logs_chart }} from \"tools/mcp/ologs.js\"`. `tools[name]` and identifier wrappers like `await shell(args)` remain available for compatibility when the tool name is a valid JS identifier. Nested tool calls resolve to their code-mode result values. Import `{{ output_text, output_image, set_max_output_tokens_per_exec_call }}` from `\"@openai/code_mode\"`; `output_text(value)` surfaces text back to the model and stringifies non-string objects when possible, `output_image(imageUrl)` appends an `input_image` content item for `http(s)` or `data:` URLs, and `set_max_output_tokens_per_exec_call(value)` sets the token budget used to truncate the final Rust-side result of the current `code_mode` execution. The default is `10000`. This guards the overall `code_mode` output, not individual nested tool invocations. When truncation happens, the final text uses the unified-exec style `Original token count:` / `Output:` wrapper and the usual `…N tokens truncated…` marker. Function tools require JSON object arguments. Freeform tools require raw strings. `add_content(value)` remains available for compatibility with a content item, content-item array, or string. Structured nested-tool results should be converted to text first, for example with `JSON.stringify(...)`. Only content passed to `output_text(...)`, `output_image(...)`, or `add_content(value)` is surfaced back to the model. Enabled nested tools: {enabled_list}."
     );
 
     ToolSpec::Freeform(FreeformTool {