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/tests/exec_server.rs

@@ -90,6 +90,24 @@ fn local_exec_server_ignores_invalid_config_without_strict_config() -> Result<()
     Ok(())
 }
 
+/// The standalone exec-server accepts an explicit per-connection concurrency limit.
+#[test]
+fn local_exec_server_accepts_concurrent_requests_flag() -> Result<()> {
+    let codex_home = TempDir::new()?;
+    let mut cmd = codex_command(codex_home.path())?;
+    cmd.args([
+        "exec-server",
+        "--listen",
+        "stdio",
+        "--concurrent-requests",
+        "2",
+    ])
+    .assert()
+    .success();
+
+    Ok(())
+}
+
 #[test]
 fn local_exec_server_allows_disabled_parent_lifetime_environment_variable() -> Result<()> {
     let codex_home = TempDir::new()?;
@@ -173,6 +191,8 @@ metrics_exporter = {{ otlp-http = {{ endpoint = "{collector_url}/v1/metrics", pr
             &registry.uri(),
             "--environment-id",
             ENVIRONMENT_ID,
+            "--concurrent-requests",
+            "2",
         ])
         .stdin(Stdio::piped())
         .stderr(Stdio::piped())