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/src/codex_message_processor.rs

@@ -1186,6 +1186,11 @@ impl CodexMessageProcessor {
                 self.command_exec_terminate(to_connection_request_id(request_id), params)
                     .await;
             }
+            ClientRequest::DeviceKeyCreate { .. }
+            | ClientRequest::DeviceKeyPublic { .. }
+            | ClientRequest::DeviceKeySign { .. } => {
+                warn!("Device key request reached CodexMessageProcessor unexpectedly");
+            }
             ClientRequest::ConfigRead { .. }
             | ClientRequest::ConfigValueWrite { .. }
             | ClientRequest::ConfigBatchWrite { .. }