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/process_metrics.rs

@@ -0,0 +1,21 @@
+//! Optional process observations, independent of sessions. Process-owned
+//! providers observe existing children; remote transports own connection policy.
+
+/// A cumulative observation of one host process, shared across its sessions.
+#[derive(Clone, Debug, PartialEq)]
+pub struct CodeModeProcessMetrics {
+    pub process_id: u32,
+    /// Unix timestamp, stable for this process across all metrics transports.
+    pub process_start_time_seconds: f64,
+    pub cpu_seconds_total: f64,
+    pub resident_memory_bytes: u64,
+}
+
+/// Unsupported capability and an absent process are distinct from a failed sample.
+#[derive(Clone, Debug, PartialEq)]
+pub enum CodeModeProcessMetricsSample {
+    Unsupported,
+    /// The process owner knows no host is running; never inferred from an RPC status.
+    NotRunning,
+    Snapshot(CodeModeProcessMetrics),
+}