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/src/session/mod.rs

@@ -1931,6 +1931,18 @@ impl Session {
             config.mcp_servers = next_config.mcp_servers.clone();
             config.mcp_optional_startup_grace = next_config.mcp_optional_startup_grace;
             config.mcp_oauth_credentials_store_mode = next_config.mcp_oauth_credentials_store_mode;
+            if let Err(err) = config.features.set_enabled(
+                Feature::Mcp20260728,
+                next_config.features.enabled(Feature::Mcp20260728),
+            ) {
+                warn!("failed to refresh MCP protocol config: {err}");
+            }
+            if let Err(err) = config.features.set_enabled(
+                Feature::CodexAppsMcp20260728,
+                next_config.features.enabled(Feature::CodexAppsMcp20260728),
+            ) {
+                warn!("failed to refresh Codex Apps MCP protocol config: {err}");
+            }
             if let Err(err) = config.features.set_enabled(
                 Feature::SecretAuthStorage,
                 next_config.features.enabled(Feature::SecretAuthStorage),
@@ -1987,6 +1999,18 @@ impl Session {
         config.mcp_servers = next_config.mcp_servers;
         config.mcp_optional_startup_grace = next_config.mcp_optional_startup_grace;
         config.mcp_oauth_credentials_store_mode = next_config.mcp_oauth_credentials_store_mode;
+        if let Err(err) = config.features.set_enabled(
+            Feature::Mcp20260728,
+            next_config.features.enabled(Feature::Mcp20260728),
+        ) {
+            warn!("failed to refresh MCP protocol config: {err}");
+        }
+        if let Err(err) = config.features.set_enabled(
+            Feature::CodexAppsMcp20260728,
+            next_config.features.enabled(Feature::CodexAppsMcp20260728),
+        ) {
+            warn!("failed to refresh Codex Apps MCP protocol config: {err}");
+        }
         if let Err(err) = config.features.set_enabled(
             Feature::SecretAuthStorage,
             next_config.features.enabled(Feature::SecretAuthStorage),