Give hosted Codex Apps an independent MCP protocol opt-in (#44318)

## Why

Hosted Codex Apps shared the `mcp_2026_07_28` protocol setting with other MCP servers, preventing independent protocol selection.

## What changed

- Add `features.codex_apps_mcp_2026_07_28`, disabled by default, for the host-owned HTTP `codex_apps` server. Enabling it attempts 2026-07-28 discovery with Legacy fallback.
- Keep other servers, including third-party registrations with Apps-like names or URLs and local stdio servers, governed by `mcp_2026_07_28`.
- Support the new flag through `experimentalFeature/enablement/set` and refresh protocol settings in existing sessions. Track protocol mode per server when deciding whether to reuse connections.

## Testing

Add coverage for independent feature and protocol selection, hosted event-stream protocol selection, standalone Legacy defaults and discovery fallback, and runtime enablement refreshing an existing app-server thread.

GitOrigin-RevId: 640e8e251f0c45e5f4829cc2b73c3082cf42f715

codex-rs/core/config.schema.json

@@ -701,6 +701,9 @@
             "code_mode_prewarm": {
               "type": "boolean"
             },
+            "codex_apps_mcp_2026_07_28": {
+              "type": "boolean"
+            },
             "codex_git_commit": {
               "type": "boolean"
             },
@@ -6190,6 +6193,9 @@
         "code_mode_prewarm": {
           "type": "boolean"
         },
+        "codex_apps_mcp_2026_07_28": {
+          "type": "boolean"
+        },
         "codex_git_commit": {
           "type": "boolean"
         },