Add new apps_mcp_gateway (#11630)

Adds a new apps_mcp_gateway flag to route Apps MCP calls through
https://api.openai.com/v1/connectors/mcp/ when enabled, while keeping
legacy MCP routing as default.

codex-rs/core/src/features.rs

@@ -121,6 +121,8 @@ pub enum Feature {
     Collab,
     /// Enable apps.
     Apps,
+    /// Route apps MCP calls through the configured gateway.
+    AppsMcpGateway,
     /// Allow prompting and installing missing MCP dependencies.
     SkillMcpDependencyInstall,
     /// Prompt for missing skill env var dependencies.
@@ -566,6 +568,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         },
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::AppsMcpGateway,
+        key: "apps_mcp_gateway",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::SkillMcpDependencyInstall,
         key: "skill_mcp_dependency_install",