feat(exec-server): use protobuf relay frames (#22343)

## Why

Remote exec-server now needs one executor websocket to serve multiple
harness JSON-RPC sessions. Rendezvous routes by `stream_id`, and the
exec-server side needs to use the same stable relay frame contract
instead of a hand-rolled JSON shape.

The relay protocol also needs to make ownership boundaries clear:
harness and executor endpoints own sequencing, acks, retries, duplicate
suppression, segmentation, and reassembly; rendezvous only routes
frames.

## What Changed

- Add the checked-in `codex.exec_server.relay.v1.RelayMessageFrame`
proto plus generated prost bindings for `codex-exec-server`.
- Encode remote harness/executor relay traffic as binary protobuf
websocket frames while keeping local websocket JSON-RPC unchanged.
- Demux executor-side relay streams into independent
`ConnectionProcessor` sessions keyed by `stream_id`.
- Add a programmatic `RemoteExecutorConfig::with_bearer_token(...)`
constructor for non-CLI callers and integration tests.
- Add an integration test that starts the remote executor against a fake
registry/rendezvous websocket and verifies two virtual streams share one
executor websocket without cross-talk, including per-stream reset
behavior.
- Document the remote relay envelope, sequence ranges, `ack`/`ack_bits`,
and endpoint responsibilities in `exec-server/README.md`.

## Verification

- `cargo test -p codex-exec-server --test relay
multiplexed_remote_executor_routes_independent_virtual_streams --
--exact`
- `cargo test -p codex-exec-server --test relay`
- `cargo test -p codex-exec-server` passed outside the sandbox. The
sandboxed run hit macOS `sandbox-exec: sandbox_apply: Operation not
permitted` in filesystem sandbox tests.

codex-rs/exec-server/src/remote.rs

@@ -11,7 +11,7 @@ use codex_utils_rustls_provider::ensure_rustls_crypto_provider;
 
 use crate::ExecServerError;
 use crate::ExecServerRuntimePaths;
-use crate::connection::JsonRpcConnection;
+use crate::relay::run_multiplexed_executor;
 use crate::server::ConnectionProcessor;
 
 pub const CODEX_EXEC_SERVER_REMOTE_BEARER_TOKEN_ENV_VAR: &str =
@@ -113,7 +113,7 @@ impl RemoteExecutorConfig {
         Self::with_bearer_token(base_url, executor_id, read_remote_bearer_token_from_env()?)
     }
 
-    fn with_bearer_token(
+    pub fn with_bearer_token(
         base_url: String,
         executor_id: String,
         bearer_token: String,
@@ -150,12 +150,7 @@ pub async fn run_remote_executor(
         match connect_async(response.url.as_str()).await {
             Ok((websocket, _)) => {
                 backoff = Duration::from_secs(1);
-                processor
-                    .run_connection(JsonRpcConnection::from_websocket(
-                        websocket,
-                        "remote exec-server websocket".to_string(),
-                    ))
-                    .await;
+                run_multiplexed_executor(websocket, processor.clone()).await;
             }
             Err(err) => {
                 warn!("failed to connect remote exec-server websocket: {err}");