Return public key metadata from user verification enrollment (#44877)

## Why

The trusted UI host needs the local credential's public metadata to complete backend registration. `userVerification/enroll` previously returned only `credentialId`.

## What changed

- Return `algorithm` and `publicKey` for newly created or reused credentials. The algorithm is `ecdsaP256Sha256X962`; the public key is unpadded base64url SPKI-DER.
- Keep both fields optional in the protocol for compatibility with older app-servers, while current servers populate both.
- Document caller-owned backend registration and revocation, including checking metadata, signing an enrollment challenge with `userVerification/verify`, matching credential IDs, and preserving the authenticated account throughout registration.

## Testing

Add protocol coverage for absent or null metadata and populated-response round trips. Add an RPC assertion for enrollment metadata and extend the local enrollment test to check metadata when creating and reusing a key.

GitOrigin-RevId: f0726e8c430e27559e1a01ba2ea635993cbeba09

codex-rs/app-server-protocol/src/protocol/v2/user_verification_tests.rs

@@ -2,6 +2,35 @@ use super::*;
 use pretty_assertions::assert_eq;
 use serde_json::json;
 
+#[test]
+fn enrollment_response_accepts_older_servers_without_public_metadata() {
+    for response in [
+        json!({"credentialId": "credential"}),
+        json!({"credentialId": "credential", "algorithm": null, "publicKey": null}),
+    ] {
+        assert_eq!(
+            serde_json::from_value::<UserVerificationEnrollResponse>(response).unwrap(),
+            UserVerificationEnrollResponse {
+                credential_id: "credential".into(),
+                algorithm: None,
+                public_key: None,
+            }
+        );
+    }
+    let response = UserVerificationEnrollResponse {
+        credential_id: "credential".into(),
+        algorithm: Some("ecdsaP256Sha256X962".into()),
+        public_key: Some("public-key".into()),
+    };
+    assert_eq!(
+        serde_json::from_value::<UserVerificationEnrollResponse>(
+            serde_json::to_value(&response).unwrap()
+        )
+        .unwrap(),
+        response
+    );
+}
+
 #[test]
 fn local_readiness_retains_credential_during_biometric_unavailability() {
     let status = UserVerificationStatusResponse {