Support remote code-mode hosts in app-server (#35098)

## What changed

- Add `--code-mode-host ws://...` and `wss://...` support to `codex app-server`, gated by the `code_mode_host` feature. When omitted, app-server continues to start a local host.
- Share one remote WebSocket connection across the process's threads, using the configured HTTP client's proxy and TLS policy and preserving the existing framed host protocol.
- Reject invalid host URLs, bound WebSocket frame sizes, close connections cleanly, and return an error when a connection exceeds 1,024 pending delegate calls without disconnecting it.

## Testing

- Cover CLI validation, WebSocket protocol execution and shutdown, connection sharing across app-server threads, and delegate-call capacity recovery.

GitOrigin-RevId: 715e82d4d9db1e7e2f91b754a777dcab504e2ae4

codex-rs/code-mode-host/src/peer.rs

@@ -13,6 +13,7 @@ use codex_code_mode_protocol::host::DelegateRequestId;
 use codex_code_mode_protocol::host::DelegateResponse;
 use codex_code_mode_protocol::host::EncodedFrame;
 use codex_code_mode_protocol::host::HostToClient;
+use codex_code_mode_protocol::host::MAX_PENDING_DELEGATE_CALLS;
 use codex_code_mode_protocol::host::RequestId;
 use codex_code_mode_protocol::host::SessionId;
 use codex_code_mode_protocol::host::WireResult;
@@ -25,7 +26,6 @@ use tokio::sync::oneshot;
 use tokio_util::sync::CancellationToken;
 
 const CELL_MESSAGE_CAPACITY: usize = 128;
-const MAX_PENDING_DELEGATE_CALLS: usize = 256;
 
 pub(super) struct HostPeer {
     outgoing_tx: mpsc::Sender<EncodedFrame>,