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/core/src/session/step_context.rs

@@ -62,11 +62,13 @@ impl StepContext {
         if self.settings.model_info.use_responses_lite {
             Some(Feature::IncrementalTools)
         } else {
-            matches!(
-                self.turn.config.model_provider_id.as_str(),
-                AMAZON_BEDROCK_PROVIDER_ID | AMAZON_BEDROCK_RUNTIME_PROVIDER_ID | GCP_PROVIDER_ID
-            )
-            .then_some(Feature::IncrementalToolsLegacy)
+            match self.turn.config.model_provider_id.as_str() {
+                AMAZON_BEDROCK_PROVIDER_ID | AMAZON_BEDROCK_RUNTIME_PROVIDER_ID => {
+                    Some(Feature::IncrementalToolsLegacy)
+                }
+                GCP_PROVIDER_ID => Some(Feature::IncrementalToolsLegacyGcp),
+                _ => None,
+            }
         }
     }