Add capability-gated MCP user-verification handling (#43289)

## What changed

- Handle `openai/userVerification` requests through `openai/elicitation/create` when the client advertises `userVerification` support.
- Validate request fields, size limits, and base64url encoding. Require a correctly shaped, bounded proof for acceptance; cancel invalid acceptance responses and discard proof material on decline or cancellation.
- Handle request and service cancellation, including cancellation received before request dispatch. Bound early-cancellation storage and cancel new verifications when capacity is exceeded.
- Keep verification inactive in Codex: cancel requests without prompting until the typed app-server and UI path is available.

## Testing

Add tests for request and proof validation, malformed modes, cancellation races and capacity limits, timeout-pause cleanup, and independent concurrent requests. Add integration coverage showing that verification does not prompt or prevent tool and turn completion before activation.

GitOrigin-RevId: 6313c1be40fae0b33cc3ce0f8fe025b4124ed2ef

codex-rs/core/src/session/mcp.rs

@@ -1023,7 +1023,9 @@ fn guardian_elicitation_review_request(
                 "guardian MCP elicitation review does not support this elicitation mode",
             );
         }
-        Elicitation::OpenAiForm { .. } | Elicitation::OpenAiElicitationForm { .. } => {
+        Elicitation::OpenAiForm { .. }
+        | Elicitation::OpenAiElicitationForm { .. }
+        | Elicitation::UserVerification { .. } => {
             return GuardianElicitationReview::NotRequested;
         }
     };