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/app-server/src/request_processors/config_processor.rs

@@ -50,6 +50,7 @@ use std::path::PathBuf;
 
 const SUPPORTED_EXPERIMENTAL_FEATURE_ENABLEMENT: &[&str] = &[
     "auth_elicitation",
+    "mcp_2026_07_28",
     "memories",
     "mentions_v2",
     "remote_control",