Add opt-in gRPC over stdio for the code-mode host (#52723)

## What changed

- Add `grpc+stdio://` as a host transport and the disabled-by-default `code_mode_host_grpc` feature flag to select it.
- Share a lazy HTTP/2 channel and host process across code-mode sessions while keeping session state isolated.
- Use HTTP/2 keepalives to detect unresponsive hosts, kill and reap disconnected children, and spawn a fresh host on reconnect. Exit the host when its stdio connection closes.

## Testing

Add coverage for shared-host session isolation, host exit on stdin EOF, and recovery after an unresponsive host is reaped. Exercise nested tool calls with both legacy stdio and gRPC stdio, asserting that the feature flag selects the expected transport.

GitOrigin-RevId: 31389d8540de236b8e0571d3c24e29e117b75c4a

codex-rs/features/src/lib.rs

@@ -133,6 +133,8 @@ pub enum Feature {
     CodeModeBufferedExec,
     /// Run JavaScript code mode in the standalone host process.
     CodeModeHost,
+    /// Use gRPC over stdio for the process-owned code-mode host.
+    CodeModeHostGrpc,
     /// Establish the code-mode host connection during session startup.
     CodeModePrewarm,
     /// Terminate active code mode cells when their turn is interrupted.
@@ -1162,6 +1164,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::CodeModeHostGrpc,
+        key: "code_mode_host_grpc",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CodeModePrewarm,
         key: "code_mode_prewarm",