Add experimental user verification API contracts (#43265)

## What changed

Add `userVerification/status`, `userVerification/enroll`, `userVerification/delete`, and `userVerification/verify` behind the `experimentalApi` capability. Define request, response, proof, and typed error contracts, export their schemas, and document the API.

All four methods currently return `unavailable` with reason `providerUnavailable`; native verification operations are not implemented by this change.

## Testing

Add tests for experimental opt-in, typed provider unavailability, readiness serialization, and rejection of mismatched error reasons and native diagnostic payloads.

GitOrigin-RevId: a858ab41523c5acec7dbf3cfa7fcab693ac44a8c

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

@@ -33,6 +33,7 @@ mod thread;
 mod thread_data;
 mod thread_usage;
 mod turn;
+mod user_verification;
 mod windows_sandbox;
 
 pub use account::*;
@@ -69,6 +70,7 @@ pub use thread::*;
 pub use thread_data::*;
 pub use thread_usage::*;
 pub use turn::*;
+pub use user_verification::*;
 pub use windows_sandbox::*;
 
 #[cfg(test)]