[codex] Skip plugin MCP OAuth for matching app routes (#27461)

## Context

This is PR5 in the plugin auth-routing stack. Earlier PRs make plugin
surface projection auth-aware, narrow App/MCP conflicts by App
declaration name, and keep connector listings auth-aware. This PR
applies the same name-based App/MCP conflict rule into plugin MCP
loading, so install-time MCP OAuth and plugin detail metadata both
reflect the MCPs available for the current auth route.

## Stack

- PR1: #27652 seed plugin manager auth at construction.
- PR2: #27459 route plugin surfaces by auth mode.
- PR3: #27607 dedupe plugin MCP servers by App declaration name.
- PR4: #27602 preserve plugin Apps in connector listings.
- PR5: #27461 skip install-time plugin MCP OAuth for matching App
routes.

## Summary

- Make `load_plugin_mcp_servers` auth-aware and let it load App
declarations before filtering same-name MCP servers for Codex-backend
auth.
- Use that filtered MCP list for both install-time MCP OAuth and
marketplace plugin detail metadata.
- Preserve API-key/direct auth behavior so plugin MCP servers remain
visible and can still start OAuth.

## Validation

```bash
cargo fmt --all
cargo test -p codex-core-plugins read_plugin_for_config_filters_mcp_servers_for_codex_backend_auth
cargo check -p codex-core-plugins -p codex-app-server
git diff --check
git diff --cached --check
```

codex-rs/core-plugins/src/loader.rs

@@ -10,6 +10,7 @@ use crate::remote::REMOTE_GLOBAL_MARKETPLACE_NAME;
 use crate::remote::RemoteInstalledPlugin;
 use crate::store::PluginStore;
 use crate::store::plugin_version_for_source;
+use codex_app_server_protocol::AuthMode;
 use codex_config::ConfigLayerStack;
 use codex_config::HooksFile;
 use codex_config::types::McpServerConfig;
@@ -1079,7 +1080,29 @@ pub async fn plugin_telemetry_metadata_from_root(
     }
 }
 
-pub async fn load_plugin_mcp_servers(plugin_root: &Path) -> HashMap<String, McpServerConfig> {
+pub async fn load_plugin_mcp_servers(
+    plugin_root: &Path,
+    auth_mode: Option<AuthMode>,
+) -> HashMap<String, McpServerConfig> {
+    let mut mcp_servers = load_declared_plugin_mcp_servers(plugin_root).await;
+    if !auth_mode.is_some_and(AuthMode::uses_codex_backend) || mcp_servers.is_empty() {
+        return mcp_servers;
+    }
+
+    let app_declarations = load_plugin_apps(plugin_root).await;
+    if app_declarations.is_empty() {
+        return mcp_servers;
+    }
+
+    let app_declaration_names = app_declarations
+        .iter()
+        .map(|app| app.name.as_str())
+        .collect::<HashSet<_>>();
+    mcp_servers.retain(|name, _| !app_declaration_names.contains(name.as_str()));
+    mcp_servers
+}
+
+async fn load_declared_plugin_mcp_servers(plugin_root: &Path) -> HashMap<String, McpServerConfig> {
     let Some(manifest) = load_plugin_manifest(plugin_root) else {
         return HashMap::new();
     };

codex-rs/core-plugins/src/manager.rs

@@ -1330,7 +1330,7 @@ impl PluginsManager {
                 app_category_by_id.insert(app.connector_id.0.clone(), category.clone());
             }
         }
-        let mut mcp_server_names = load_plugin_mcp_servers(source_path.as_path())
+        let mut mcp_server_names = load_plugin_mcp_servers(source_path.as_path(), self.auth_mode())
             .await
             .into_keys()
             .collect::<Vec<_>>();

codex-rs/core-plugins/src/manager_tests.rs

@@ -2425,6 +2425,82 @@ enabled = true
     assert!(matches!(err, MarketplaceError::PluginsDisabled));
 }
 
+#[tokio::test]
+async fn read_plugin_for_config_filters_mcp_servers_for_codex_backend_auth() {
+    let tmp = tempfile::tempdir().unwrap();
+    let repo_root = tmp.path().join("repo");
+    fs::create_dir_all(repo_root.join(".git")).unwrap();
+    fs::create_dir_all(repo_root.join(".agents/plugins")).unwrap();
+    write_file(
+        &repo_root.join(".agents/plugins/marketplace.json"),
+        r#"{
+  "name": "debug",
+  "plugins": [
+    {
+      "name": "sample-plugin",
+      "source": {
+        "source": "local",
+        "path": "./sample-plugin"
+      }
+    }
+  ]
+}"#,
+    );
+    write_file(
+        &repo_root.join("sample-plugin/.codex-plugin/plugin.json"),
+        r#"{"name":"sample-plugin"}"#,
+    );
+    write_file(
+        &repo_root.join("sample-plugin/.app.json"),
+        r#"{"apps":{"sample-mcp":{"id":"connector_sample"}}}"#,
+    );
+    write_file(
+        &repo_root.join("sample-plugin/.mcp.json"),
+        r#"{"mcpServers":{"other-mcp":{"command":"other-mcp"},"sample-mcp":{"command":"sample-mcp"}}}"#,
+    );
+    write_file(
+        &tmp.path().join(CONFIG_TOML_FILE),
+        r#"[features]
+plugins = true
+"#,
+    );
+
+    let config = load_config(tmp.path(), &repo_root).await;
+    let request = PluginReadRequest {
+        plugin_name: "sample-plugin".to_string(),
+        marketplace_path: AbsolutePathBuf::try_from(
+            repo_root.join(".agents/plugins/marketplace.json"),
+        )
+        .unwrap(),
+    };
+
+    let chatgpt_outcome = PluginsManager::new_with_options(
+        tmp.path().to_path_buf(),
+        Some(Product::Codex),
+        Some(AuthMode::Chatgpt),
+    )
+    .read_plugin_for_config(&config, &request)
+    .await
+    .unwrap();
+    assert_eq!(
+        chatgpt_outcome.plugin.mcp_server_names,
+        vec!["other-mcp".to_string()]
+    );
+
+    let api_key_outcome = PluginsManager::new_with_options(
+        tmp.path().to_path_buf(),
+        Some(Product::Codex),
+        Some(AuthMode::ApiKey),
+    )
+    .read_plugin_for_config(&config, &request)
+    .await
+    .unwrap();
+    assert_eq!(
+        api_key_outcome.plugin.mcp_server_names,
+        vec!["other-mcp".to_string(), "sample-mcp".to_string()]
+    );
+}
+
 #[tokio::test]
 async fn read_plugin_for_config_uses_user_layer_skill_settings_only() {
     let tmp = tempfile::tempdir().unwrap();