Hide app-only MCP tools from model

[arunv]

Summary

• Honor MCP Apps _meta.ui.visibility when building model-visible MCP tools.
• Keep app-only MCP tools in the raw app-server inventory so Codex app UI can still discover and call them.

Why

Plugin Runtime MCP can emit private connector actions as _meta.ui.visibility = ["app"] via openai/openai#956651. Codex app UI reads the raw MCP server inventory, but model direct tools and tool_search are also derived from MCP tools. Without this filter, app-only tools are still selectable by the model.

Validation

• Ran just test -p codex-core mcp_tool_exposure::tests::hides_app_only_tools_from_model_exposure.
• Ran just test -p codex-mcp.
• Ran just fix -p codex-core and just fix -p codex-mcp.
• Attempted just fmt; Rust cargo fmt completed, then the SDK Python Ruff step failed because this Linux environment cannot resolve openai-codex-cli-bin==0.131.0a4.
• Attempted just test -p codex-core; it failed locally on unrelated missing codex / test_stdio_server test binaries and sandbox/timeouts.

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}