Enable gRPC and host output truncation by default for Code Mode (#59973)

## What changed

Mark `code_mode_host_grpc` and `code_mode_host_output_truncation` as stable and enable them by default. Code Mode now defaults to gRPC over stdio and truncates output in the host before sending it to Core.

## Testing

Update integration tests to exercise the default configuration and explicit feature opt-outs, covering output budgets, truncation notices, and wait termination. Keep the stdio reconnection test explicitly configured for that transport.

GitOrigin-RevId: 5847a249d698fb9b1ecf4921aca4e6efc501f890

codex-rs/features/src/lib.rs

@@ -1321,14 +1321,14 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::CodeModeHostGrpc,
         key: "code_mode_host_grpc",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::CodeModeHostOutputTruncation,
         key: "code_mode_host_output_truncation",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::CodeModePrewarm,