Add cancellation for native user-verification RPCs (#43925)

## Why

Canceling an elicitation does not stop a separate native verification RPC. Clients need a way to cancel that work and prevent a canceled proof from being delivered while it waits for outbound queue capacity.

## What changed

- Add experimental `userVerification/cancel` with `{requestId}` to signal cancellation of status, enroll, delete, or verify requests on the same connection.
- Return an empty acknowledgment without waiting for native work to exit. Unknown, finished, unrelated, and other-connection requests are no-ops.
- Retain request cancellation state until outbound capacity is available so canceled proofs are checked before enqueueing. Already enqueued responses and completed effects cannot be retracted.
- Document the API, request ID requirements, and independent lifetimes of elicitations and native verification RPCs.

## Testing

Add coverage for experimental opt-in, parameter validation, connection and request ID isolation, cancellation before native dispatch, repeated cancellation, worker-slot retention, and proof cancellation while waiting for outbound capacity.

GitOrigin-RevId: 03fcb83fcc5fab4169c68668822fc0bbb4970938

codex-rs/app-server/tests/suite/v2/user_verification.rs

@@ -34,6 +34,7 @@ async fn user_verification_methods_require_experimental_opt_in() -> Result<()> {
         ("userVerification/status", json!({})),
         ("userVerification/enroll", json!({})),
         ("userVerification/delete", json!({})),
+        ("userVerification/cancel", json!({"requestId": 1})),
         (
             "userVerification/verify",
             json!({"challenge": "AQ", "title": "Approve", "description": ""}),
@@ -75,6 +76,49 @@ async fn user_verification_status_without_account_returns_local_readiness() -> R
     Ok(())
 }
 
+#[tokio::test]
+async fn user_verification_cancel_stdio_validates_and_acknowledges_unknown_requests() -> Result<()>
+{
+    let mut app_server = TestAppServer::builder().build().await?;
+    let id = app_server
+        .send_raw_request(
+            "userVerification/cancel",
+            Some(json!({"requestId": "verify"})),
+        )
+        .await?;
+    let error = app_server
+        .read_stream_until_error_message(RequestId::Integer(id))
+        .await?;
+    assert_eq!(error.error.message, "Not initialized");
+    app_server.initialize().await?;
+    for params in [
+        json!({}),
+        json!({"requestId": null}),
+        json!({"requestId": 1.5}),
+        json!({"requestId": 1, "connectionId": 2}),
+    ] {
+        let id = app_server
+            .send_raw_request("userVerification/cancel", Some(params))
+            .await?;
+        app_server
+            .read_stream_until_error_message(RequestId::Integer(id))
+            .await?;
+    }
+    for request_id in [json!(100), json!("100"), json!("100")] {
+        let id = app_server
+            .send_raw_request(
+                "userVerification/cancel",
+                Some(json!({"requestId": request_id})),
+            )
+            .await?;
+        let response = app_server
+            .read_stream_until_response_message(RequestId::Integer(id))
+            .await?;
+        assert_eq!(response.result, json!({}));
+    }
+    Ok(())
+}
+
 #[tokio::test]
 async fn user_verification_without_provider_returns_typed_unavailability() -> Result<()> {
     let mut app_server = TestAppServer::builder().build_initialized().await?;