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/config/config_tests.rs

@@ -7018,7 +7018,7 @@ async fn to_mcp_config_flows_mcp_tool_prefix_from_feature() -> std::io::Result<(
 }
 
 #[tokio::test]
-async fn to_mcp_config_flows_mcp_2026_feature_from_config() -> std::io::Result<()> {
+async fn to_mcp_config_flows_independent_mcp_2026_features() -> std::io::Result<()> {
     let codex_home = TempDir::new()?;
     let mut config = Config::load_from_base_config_with_overrides(
         ConfigToml::default(),
@@ -7030,13 +7030,42 @@ async fn to_mcp_config_flows_mcp_2026_feature_from_config() -> std::io::Result<(
         plugins_manager_for_config(&config, auth_manager_from_optional_auth(/*auth*/ None));
 
     let mcp_config = config.to_mcp_config(&plugins_manager).await;
-    assert_eq!(mcp_config.protocol_mode, codex_mcp::McpProtocolMode::Legacy);
+    assert_eq!(
+        (
+            mcp_config.protocol_mode,
+            mcp_config.host_owned_apps_protocol_mode,
+        ),
+        (
+            codex_mcp::McpProtocolMode::Legacy,
+            codex_mcp::McpProtocolMode::Legacy,
+        )
+    );
 
     let _ = config.features.enable(Feature::Mcp20260728);
     let mcp_config = config.to_mcp_config(&plugins_manager).await;
     assert_eq!(
-        mcp_config.protocol_mode,
-        codex_mcp::McpProtocolMode::V20260728
+        (
+            mcp_config.protocol_mode,
+            mcp_config.host_owned_apps_protocol_mode,
+        ),
+        (
+            codex_mcp::McpProtocolMode::V20260728,
+            codex_mcp::McpProtocolMode::Legacy,
+        )
+    );
+
+    let _ = config.features.disable(Feature::Mcp20260728);
+    let _ = config.features.enable(Feature::CodexAppsMcp20260728);
+    let mcp_config = config.to_mcp_config(&plugins_manager).await;
+    assert_eq!(
+        (
+            mcp_config.protocol_mode,
+            mcp_config.host_owned_apps_protocol_mode,
+        ),
+        (
+            codex_mcp::McpProtocolMode::Legacy,
+            codex_mcp::McpProtocolMode::V20260728,
+        )
     );
 
     Ok(())

codex-rs/core/src/config/mod.rs

@@ -1792,6 +1792,11 @@ impl Config {
                 Vec::new()
             },
             protocol_mode: self.mcp_protocol_mode(),
+            host_owned_apps_protocol_mode: if self.features.enabled(Feature::CodexAppsMcp20260728) {
+                McpProtocolMode::V20260728
+            } else {
+                McpProtocolMode::Legacy
+            },
             client_elicitation_capability: if self.features.enabled(Feature::AuthElicitation) {
                 ElicitationCapability::new()
                     .with_form(FormElicitationCapability::new())