Route hosted Apps MCP through extensions (#27191)

## Stack

- Base: #27184
- This PR is the second vertical and should be reviewed against
`jif/external-plugins-1`, not `main`.

## Why

CCA is moving toward a split runtime where the orchestrator may have no
filesystem or executor, but it still needs to activate remotely hosted
plugin components. HTTP MCP servers are the simplest complete example:
they need configuration and host authentication, but they do not need an
executor process.

The Apps MCP endpoint is currently synthesized by a special-purpose
loader inside the MCP runtime. That works locally, but it leaves hosted
MCP activation outside the extension model being established in #27184.
It also makes the Apps path a poor foundation for plugins whose skills,
MCP servers, connectors, and hooks may come from different sources or
execute in different places.

This PR moves that one behavior behind an extension-owned contribution
while preserving the existing local fallback. It deliberately does not
introduce a generic plugin activation framework.

## What changed

### MCP extension contribution

`codex-extension-api` gains an ordered `McpServerContributor` contract.
A contributor returns typed `Set` or `Remove` overlays for MCP server
configuration; later contributors win for the names they own.

The contract stays at the existing MCP configuration boundary.
Extensions do not create a second connection manager or transport
abstraction.

### Hosted Apps MCP extension

A new `codex-mcp-extension` contributes the reserved `codex_apps` server
from the existing Apps feature, ChatGPT base URL, path override, and
product SKU configuration.

When `apps_mcp_path_override` is enabled for `https://chatgpt.com`, the
resulting streamable HTTP endpoint is
`https://chatgpt.com/backend-api/ps/mcp`. The existing ChatGPT-auth gate
remains authoritative, so this server can run in an orchestrator-only
process without being exposed for API-key sessions.

### One resolved runtime view

`McpManager` now distinguishes three views:

- **configured:** config- and plugin-backed servers before extension
overlays;
- **runtime:** configured servers plus host-installed extension
contributions;
- **effective:** runtime servers after auth gating and compatibility
built-ins.

App-server installs the hosted MCP extension and uses the runtime view
for thread startup, refresh, status, threadless resource reads,
connector discovery, and MCP OAuth lookup. This keeps
`mcpServer/oauth/login` consistent with the servers exposed by the other
MCP APIs. The hosted Apps server itself continues to use existing
ChatGPT host authentication rather than MCP OAuth.

## Compatibility

Hosts that do not install the MCP extension retain the existing Apps MCP
synthesis path. This preserves current local-only, CLI, and
standalone-host behavior while app-server exercises the extension path.

Disabling Apps removes the reserved `codex_apps` entry, and losing
ChatGPT auth removes it from the effective runtime view. Executor
availability is not consulted for this HTTP transport.

## Follow-ups

The next vertical will resolve a manifest-declared stdio MCP server from
an executor-selected plugin root and execute it in the environment that
owns that root. Later verticals can add backend-owned skills, connector
metadata, hooks, durable selection semantics, and incremental local
convergence without changing the component-specific runtime boundaries
introduced here.

## Verification

Focused coverage was added for:

- contributing the hosted Apps MCP at `/backend-api/ps/mcp` without an
executor;
- requiring ChatGPT auth in the effective runtime view;
- removing a reserved configured Apps server when the Apps feature is
disabled.

`cargo check -p codex-app-server -p codex-mcp-extension -p
codex-extension-api -p codex-mcp` passed. Tests and Clippy were not run
locally under the current development instruction; CI provides the full
validation pass.

codex-rs/core/src/connectors.rs

@@ -1,4 +1,3 @@
-use std::collections::HashMap;
 use std::collections::HashSet;
 use std::sync::Arc;
 use std::sync::LazyLock;
@@ -41,8 +40,8 @@ use codex_mcp::ToolInfo;
 use codex_mcp::ToolPluginProvenance;
 use codex_mcp::codex_apps_tools_cache_key;
 use codex_mcp::compute_auth_statuses;
+use codex_mcp::effective_mcp_servers;
 use codex_mcp::host_owned_codex_apps_enabled;
-use codex_mcp::with_codex_apps_mcp;
 
 const CONNECTORS_READY_TIMEOUT_ON_EMPTY_TOOLS: Duration = Duration::from_secs(30);
 
@@ -220,6 +219,23 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_environment_manager(
     config: &Config,
     force_refetch: bool,
     environment_manager: Arc<EnvironmentManager>,
+) -> anyhow::Result<AccessibleConnectorsStatus> {
+    let plugins_manager = Arc::new(PluginsManager::new(config.codex_home.to_path_buf()));
+    let mcp_manager = Arc::new(McpManager::new(plugins_manager));
+    list_accessible_connectors_from_mcp_tools_with_mcp_manager(
+        config,
+        force_refetch,
+        environment_manager,
+        mcp_manager,
+    )
+    .await
+}
+
+pub async fn list_accessible_connectors_from_mcp_tools_with_mcp_manager(
+    config: &Config,
+    force_refetch: bool,
+    environment_manager: Arc<EnvironmentManager>,
+    mcp_manager: Arc<McpManager>,
 ) -> anyhow::Result<AccessibleConnectorsStatus> {
     let auth_manager =
         AuthManager::shared_from_config(config, /*enable_codex_api_key_env*/ false).await;
@@ -234,8 +250,6 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_environment_manager(
         });
     }
     let cache_key = accessible_connectors_cache_key(config, auth.as_ref());
-    let plugins_manager = Arc::new(PluginsManager::new(config.codex_home.to_path_buf()));
-    let mcp_manager = McpManager::new(Arc::clone(&plugins_manager));
     let tool_plugin_provenance = mcp_manager.tool_plugin_provenance(config).await;
     if !force_refetch && let Some(cached_connectors) = read_cached_accessible_connectors(&cache_key)
     {
@@ -250,8 +264,9 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_environment_manager(
         });
     }
 
-    let mcp_config = config.to_mcp_config(plugins_manager.as_ref()).await;
-    let mcp_servers = with_codex_apps_mcp(HashMap::new(), auth.as_ref(), &mcp_config);
+    let mcp_config = mcp_manager.runtime_config(config).await;
+    let mut mcp_servers = effective_mcp_servers(&mcp_config, auth.as_ref());
+    mcp_servers.retain(|name, _| name == CODEX_APPS_MCP_SERVER_NAME);
     let host_owned_codex_apps_enabled = host_owned_codex_apps_enabled(&mcp_config, auth.as_ref());
     if mcp_servers.is_empty() {
         return Ok(AccessibleConnectorsStatus {