Collect plugin metrics from remote executors (#38283)

## What changed

- Resolve manifest-declared metric operations against the executor filesystem for remote plugin commands.
- Create the measurement sidecar in an executor-native, owner-private temporary directory, stream its bounded output back for validation, and clean up the directory afterward.
- Extend the exec-server protocol with the executor temporary directory and atomic private-directory creation.

## Testing

- Cover remote unified-exec measurements for foreground and background commands.
- Verify private directories use owner-only permissions on Unix and fail closed on unsupported platforms.

GitOrigin-RevId: dbbd0a84717b91237fc5728e510e18994eb46dd4

codex-rs/core/src/unified_exec/async_watcher.rs

@@ -216,7 +216,8 @@ pub(crate) fn spawn_exit_watcher(
                 &session_ref,
                 &turn_ref,
                 &call_id,
-            );
+            )
+            .await;
             emit_exec_end_for_unified_exec(
                 session_ref,
                 turn_ref,

codex-rs/core/src/unified_exec/process_manager.rs

@@ -251,14 +251,15 @@ struct InitialExecCommandGuard {
 }
 
 impl InitialExecCommandGuard {
-    fn finish_plugin_metrics(&mut self, context: &UnifiedExecContext, exit_code: i32) {
+    async fn finish_plugin_metrics(&mut self, context: &UnifiedExecContext, exit_code: i32) {
         finish_and_track_measurements(
             self.metrics_sidecar.take(),
             exit_code,
             &context.session,
             &context.step_context.turn,
             &context.call_id,
-        );
+        )
+        .await;
     }
 }
 
@@ -661,7 +662,8 @@ impl UnifiedExecProcessManager {
                         &context.session,
                         &context.step_context.turn,
                         &context.call_id,
-                    );
+                    )
+                    .await;
                     (None, exit_code)
                 }
                 ProcessStatus::Unknown => {
@@ -694,7 +696,9 @@ impl UnifiedExecProcessManager {
             }
             let exit_code = process.exit_code();
             let exit = exit_code.unwrap_or(-1);
-            initial_exec_command_guard.finish_plugin_metrics(context, exit);
+            initial_exec_command_guard
+                .finish_plugin_metrics(context, exit)
+                .await;
             emit_exec_end_for_unified_exec(
                 Arc::clone(&context.session),
                 Arc::clone(&context.step_context.turn),