feat(app-server): add remote control pairing status RPC (#26450)

## What

Exposes the pairing status transport as experimental app-server v2 RPC
`remoteControl/pairing/status`.

- Adds request/response protocol types for exactly one lookup key:
`pairingCode` or `manualPairingCode`, returning `{ claimed }`.
- Registers the RPC with `global_shared_read("remote-control-pairing")`.
- Wires the method through `MessageProcessor` and
`RemoteControlRequestProcessor`.
- Validates missing/conflicting pairing-code params as invalid requests.
- Documents the RPC in `app-server/README.md`.
- Adds processor, protocol export, and JSON-RPC integration coverage for
both code paths.

## Why

This is the app-server surface the desktop app can poll while the
QR/manual pairing modal is active.

Depends on https://github.com/openai/codex/pull/26449
Related backend change: https://github.com/openai/openai/pull/990244

## Verification

- `cargo test --manifest-path app-server-protocol/Cargo.toml
remote_control`
- `cargo test --manifest-path app-server/Cargo.toml remote_control`
- `cargo fmt --all --check`
- `git diff --check`

codex-rs/app-server/src/message_processor.rs

@@ -921,6 +921,11 @@ impl MessageProcessor {
                 .pairing_start(params, app_server_client_name.as_deref())
                 .await
                 .map(|response| Some(response.into())),
+            ClientRequest::RemoteControlPairingStatus { params, .. } => self
+                .remote_control_processor
+                .pairing_status(params)
+                .await
+                .map(|response| Some(response.into())),
             ClientRequest::RemoteControlClientsList { params, .. } => self
                 .remote_control_processor
                 .clients_list(params)