default enable compression, update test helpers (#10102)

set `enable_request_compression` flag to default-enabled.

update integration test helpers to decompress `zstd` if flag set.

codex-rs/core/src/features.rs

@@ -506,8 +506,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::EnableRequestCompression,
         key: "enable_request_compression",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::Collab,