Feature request: make MCP apps work end-to-end in the Codex GUI

Problem

MCP apps do not work end-to-end in the Codex GUI.

A completed MCP tool call can expose mcpAppResourceUri, but the completed item does not include the hydrated resource contents the GUI needs to render the app inline. As a result, the UI has the URI but still has to do extra orchestration to display the MCP app.

Prototype

I implemented a clean prototype in my fork here:

• commit: https://github.com/vozer/codex/commit/03b480d5abf89fa954b97941d57540b6976123cf
• compare: https://github.com/openai/codex/compare/main...vozer:fix/21019-mcp-apps-clean?expand=1

What the prototype changes

• adds optional mcpAppResourceContents to completed MCP tool-call items
• hydrates the MCP app resource server-side after a successful tool call
• gates hydration behind features.enable_mcp_apps
• keeps mcpServer/resource/read available as the fallback/manual path
• keeps hydration failures non-fatal so a successful tool call does not become a failed one

Why this is the right direction

This keeps the protocol change additive, preserves the existing manual resource-read API, and gives the GUI the payload it actually needs to render MCP apps inline from completed tool calls.