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/code-mode-protocol/src/session.rs

@@ -223,6 +223,16 @@ pub trait CodeModeSessionProvider: Send + Sync {
         Ok(())
     }
 
+    /// Observes a host without starting a session. Process-owned providers must
+    /// not start or replace a host. Network providers may connect or reconnect
+    /// according to their transport policy. Sampling failures and cancellation
+    /// must not explicitly reset the shared execution connection.
+    fn sample_process_metrics(
+        &self,
+    ) -> CodeModeSessionResultFuture<'_, crate::CodeModeProcessMetricsSample> {
+        Box::pin(async { Ok(crate::CodeModeProcessMetricsSample::Unsupported) })
+    }
+
     fn create_session(&self) -> CodeModeSessionProviderFuture<'_>;
 
     /// Creates a session whose cells share the supplied execution limits.