app-server: define device key v2 protocol (#18428)

## Why

Clients need a stable app-server protocol surface for enrolling a local
device key, retrieving its public key, and producing a device-bound
proof.

The protocol reports `protectionClass` explicitly so clients can
distinguish hardware-backed keys from an explicitly allowed OS-protected
fallback. Signing uses a tagged `DeviceKeySignPayload` enum rather than
arbitrary bytes so each signed statement is auditable at the API
boundary.

## What changed

- Added v2 JSON-RPC methods for `device/key/create`,
`device/key/public`, and `device/key/sign`.
- Added request/response types for device-key metadata, SPKI public
keys, protection classes, and ECDSA signatures.
- Added `DeviceKeyProtectionPolicy` with hardware-only default behavior
and an explicit `allow_os_protected_nonextractable` option.
- Added the initial `remoteControlClientConnection` signing payload
variant.
- Regenerated JSON Schema and TypeScript fixtures for app-server
clients.

## Stack

This is PR 1 of 4 in the device-key app-server stack.

## Validation

- `just write-app-server-schema`
- `cargo test -p codex-app-server-protocol`

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -360,6 +360,18 @@ client_request_definitions! {
         params: v2::AppsListParams,
         response: v2::AppsListResponse,
     },
+    DeviceKeyCreate => "device/key/create" {
+        params: v2::DeviceKeyCreateParams,
+        response: v2::DeviceKeyCreateResponse,
+    },
+    DeviceKeyPublic => "device/key/public" {
+        params: v2::DeviceKeyPublicParams,
+        response: v2::DeviceKeyPublicResponse,
+    },
+    DeviceKeySign => "device/key/sign" {
+        params: v2::DeviceKeySignParams,
+        response: v2::DeviceKeySignResponse,
+    },
     FsReadFile => "fs/readFile" {
         params: v2::FsReadFileParams,
         response: v2::FsReadFileResponse,