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/src/grpc_stdio.rs

@@ -20,7 +20,7 @@ use tonic::transport::server::Connected;
 
 use crate::GrpcCodeModeHost;
 
-pub(crate) async fn run() -> Result<()> {
+pub(crate) async fn run(host: GrpcCodeModeHost) -> Result<()> {
     let closed = CancellationToken::new();
     let connection = StdioConnection {
         io: tokio::io::join(tokio::io::stdin(), tokio::io::stdout()),
@@ -31,7 +31,7 @@ pub(crate) async fn run() -> Result<()> {
         .chain(futures::stream::pending());
     let server = Server::builder()
         .add_service(
-            CodeModeHostServer::new(GrpcCodeModeHost::new())
+            CodeModeHostServer::new(host)
                 .max_decoding_message_size(MAX_FRAME_BYTES)
                 .max_encoding_message_size(MAX_FRAME_BYTES),
         )