Enable code mode interruption by default (#51835)

## What changed

Mark `code_mode_interrupt` as stable and enable it by default, so interrupting a turn terminates active code mode cells and nested tool calls.

## Testing

Update the existing interruption test to use the feature default instead of explicitly enabling `CodeModeInterrupt`. It verifies that active cells and nested tools are aborted while stored session values remain available.

GitOrigin-RevId: 17be94f208713bc232e2292feb48261938254db3

codex-rs/core/tests/suite/code_mode.rs

@@ -6015,7 +6015,6 @@ async fn code_mode_interrupt_terminates_active_cells_and_nested_tools() -> Resul
         .with_extensions(Arc::new(extensions.build()))
         .with_config(|config| {
             let _ = config.features.enable(Feature::CodeMode);
-            let _ = config.features.enable(Feature::CodeModeInterrupt);
             let _ = config.features.enable(Feature::ExecutedToolCallMetadata);
         });
     let test = builder.build_with_auto_env(&server).await?;

codex-rs/features/src/lib.rs

@@ -1154,8 +1154,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::CodeModeInterrupt,
         key: "code_mode_interrupt",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::CodeModeToolDescriptionFirst,