Add optional process metrics sampling for code-mode hosts (#57841)

## What changed

- Add `GetProcessMetrics` and provider sampling APIs for process ID, start time, cumulative CPU time, and resident memory, exposed through `ThreadManager` without creating a thread or session.
- Distinguish unsupported collection, a locally absent host, and sampling failures. Process-owned providers never start or replace a host when sampling.
- Bound gRPC sampling to one second without retries or explicit connection resets. Run optional collectors on a detached worker with at most one collection in flight, so stalled collection cannot block the HTTP/2 driver or runtime shutdown.

## Testing

Add coverage for snapshots, collector errors and panics, deadlines, overload, and shutdown with a stuck collector. Extend lifecycle tests to check that sampling preserves session state and keepalive behavior without starting or restarting local hosts.

GitOrigin-RevId: eeafe275891d0464636f28a29345a87aed1a12dc

codex-rs/core/src/thread_manager.rs

@@ -715,6 +715,18 @@ impl ThreadManager {
         self
     }
 
+    /// Samples host metrics without creating a Codex thread or code-mode session.
+    /// Process-owned providers observe existing children; custom providers retain
+    /// their transport's connection policy.
+    pub async fn sample_code_mode_process_metrics(
+        &self,
+    ) -> Result<codex_code_mode::CodeModeProcessMetricsSample, String> {
+        self.state
+            .code_mode_session_provider
+            .sample_process_metrics()
+            .await
+    }
+
     /// Replaces the process-wide provider before this manager is shared with threads.
     pub fn with_code_mode_session_provider(
         mut self,