Enable incremental tool declarations by default for GCP (#60848)

## What changed

- Add the stable `incremental_tools_legacy_gcp` feature, enabled by default, to control incremental tool declarations on regular GCP Responses independently of Amazon Bedrock.
- Mark `incremental_tools_legacy` stable and keep it disabled by default for Amazon Bedrock.
- Expose the new flag in the configuration schema.

## Testing

Update integration coverage for GCP defaults and opt-out, independent provider flags, and incremental tool declarations during remote compaction.

GitOrigin-RevId: 7acf085789d44122fd2a5da97932a8b9e93e240e

codex-rs/features/src/lib.rs

@@ -310,8 +310,10 @@ pub enum Feature {
     CommandSessionsWorldState,
     /// Track top-level tool definitions in world state and emit incremental context updates.
     IncrementalTools,
-    /// Publish incremental tool declarations on regular Amazon Bedrock and GCP Responses.
+    /// Publish incremental tool declarations on regular Amazon Bedrock Responses.
     IncrementalToolsLegacy,
+    /// Publish incremental tool declarations on regular GCP Responses.
+    IncrementalToolsLegacyGcp,
     /// Suppress the model ID announcement in incremental context after a model switch.
     SuppressModelChangeAnnouncements,
     /// Preserve inherited tool declarations in MCP-disabled prediction forks.
@@ -1821,9 +1823,15 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::IncrementalToolsLegacy,
         key: "incremental_tools_legacy",
-        stage: Stage::UnderDevelopment,
+        stage: Stage::Stable,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::IncrementalToolsLegacyGcp,
+        key: "incremental_tools_legacy_gcp",
+        stage: Stage::Stable,
+        default_enabled: true,
+    },
     FeatureSpec {
         id: Feature::SuppressModelChangeAnnouncements,
         key: "suppress_model_change_announcements",