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-host/tests/grpc.rs

@@ -52,6 +52,8 @@ mod image_cancellation_tests;
 mod large_tool_delegate;
 #[path = "grpc/network_policy_tests.rs"]
 mod network_policy_tests;
+#[path = "grpc/process_metrics_tests.rs"]
+mod process_metrics_tests;
 #[path = "support/recording_delegate.rs"]
 mod recording_delegate;
 #[path = "grpc/session_recovery_tests.rs"]