Prewarm code-mode host connections at session startup (#40678)

## What changed

- Add the opt-in `code_mode_prewarm` feature to establish the code-mode host session during startup, before the first turn.
- Make in-progress host initialization cancellable so a stalled prewarm does not block shutdown.
- Share a failed connection attempt with concurrent callers instead of immediately starting another attempt.

## Testing

- Verify app-server contacts the configured host before the first turn and can shut down while that connection is stalled.
- Exercise shared remote-host behavior with prewarming enabled.

GitOrigin-RevId: 3cfde5509be7b4aba80112c36c5616b9a9b632c6

codex-rs/features/src/lib.rs

@@ -111,6 +111,8 @@ pub enum Feature {
     CodeModeBufferedExec,
     /// Run JavaScript code mode in the standalone host process.
     CodeModeHost,
+    /// Establish the code-mode host connection during session startup.
+    CodeModePrewarm,
     /// Terminate active code mode cells when their turn is interrupted.
     CodeModeInterrupt,
     /// Restrict model-visible tools to code mode entrypoints (`exec`, `wait`).
@@ -949,6 +951,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::CodeModePrewarm,
+        key: "code_mode_prewarm",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CodeModeInterrupt,
         key: "code_mode_interrupt",