Add opt-in concurrent exec-server request dispatch (#36987)

## Why

Sequential dispatch lets a long-running request block unrelated health checks
and cleanup on the same connection.

## What changed

- Add `--concurrent-requests <COUNT>` for local and remote exec-server
  connections, while retaining sequential dispatch when the option is omitted
  or set to `1`.
- Preserve handshake ordering before enabling concurrent dispatch.
- Reserve separate capacity for status, signal, terminate, and close requests so
  they remain responsive when ordinary request capacity is saturated.
- Drain queued client responses during disconnect and cancel outstanding
  request tasks during connection shutdown.

## Testing

- Cover CLI parsing and concurrency-limit validation.
- Verify default sequential behavior, pipelined handshake ordering, concurrent
  request progress, control-request responsiveness, and disconnect handling.

GitOrigin-RevId: 48e4b092e318204ed635543f01f9ee0e7df095fc

codex-rs/cli/src/main.rs

@@ -573,6 +573,14 @@ struct ExecServerCommand {
     #[arg(long = "strict-config", default_value_t = false)]
     strict_config: bool,
 
+    /// Maximum number of requests to process concurrently on each connection.
+    #[arg(
+        long = "concurrent-requests",
+        value_name = "COUNT",
+        default_value = "1"
+    )]
+    request_dispatch_mode: codex_exec_server::RequestDispatchMode,
+
     /// Transport endpoint URL. Supported values: `ws://IP:PORT` (default), `stdio`, `stdio://`.
     #[arg(long = "listen", value_name = "URL", conflicts_with = "remote")]
     listen: Option<String>,
@@ -1745,6 +1753,7 @@ async fn run_exec_server_command(
         if let Some(name) = cmd.name {
             remote_config.name = name;
         }
+        remote_config.request_dispatch_mode = cmd.request_dispatch_mode;
         let remote_config = remote_config.with_telemetry(telemetry);
         let parent_lifetime = if cmd.exit_on_stdin_close {
             exec_server_telemetry::ParentLifetime::StdinPipe
@@ -1794,6 +1803,7 @@ async fn run_exec_server_command(
                     runtime_paths,
                     telemetry,
                     http_client_factory,
+                    cmd.request_dispatch_mode,
                 )
                 .await
             },