add(core): arc_monitor (#13936)

## Summary
- add ARC monitor support for MCP tool calls by serializing MCP approval
requests into the ARC action shape and sending the relevant
conversation/policy context to the `/api/codex/safety/arc` endpoint
- route ARC outcomes back into MCP approval flow so `ask-user` falls
back to a user prompt and `steer-model` blocks the tool call, with
guardian/ARC tests covering the new request shape
- update the TUI approval copy from “Approve Once” to “Allow” / “Allow
for this session” and refresh the related
  snapshots

---------

Co-authored-by: Fouad Matin <fouad@openai.com>
Co-authored-by: Fouad Matin <169186268+fouad-openai@users.noreply.github.com>

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

@@ -229,7 +229,7 @@ pub(crate) trait Approvable<Req> {
 
     // In most cases (shell, unified_exec), a request will have a single approval key.
     //
-    // However, apply_patch needs session "approve once, don't ask again" semantics that
+    // However, apply_patch needs session "Allow, don't ask again" semantics that
     // apply to multiple atomic targets (e.g., apply_patch approves per file path). Returning
     // a list of keys lets the runtime treat the request as approved-for-session only if
     // *all* keys are already approved, while still caching approvals per-key so future