Register the MCP 2026-07-28 feature flag (#34747)

## What changed

- Add the disabled-by-default, under-development `mcp_2026_07_28` feature.
- Include the feature in the configuration schema and app-server experimental
  feature enablement API.

GitOrigin-RevId: 869835ee72f063d79ca2ecd9f2655d2efc457292

codex-rs/features/src/lib.rs

@@ -160,6 +160,8 @@ pub enum Feature {
     Apps,
     /// Enable MCP apps.
     EnableMcpApps,
+    /// Enable MCP protocol version 2026-07-28 support.
+    Mcp20260728,
     /// Removed compatibility flag for the legacy Apps MCP path override.
     AppsMcpPathOverride,
     /// Removed compatibility flag retained as a no-op now that tool_search is always enabled.
@@ -1081,6 +1083,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::Mcp20260728,
+        key: "mcp_2026_07_28",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::AppsMcpPathOverride,
         key: "apps_mcp_path_override",