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/app-server/src/request_processors/plugins.rs

@@ -1036,6 +1036,7 @@ impl PluginRequestProcessor {
                     &config,
                     &outcome.plugin.apps,
                     Arc::clone(&environment_manager),
+                    self.thread_manager.mcp_manager(),
                 )
                 .await;
                 let visible_skills = outcome
@@ -1118,6 +1119,7 @@ impl PluginRequestProcessor {
                     &config,
                     &plugin_apps,
                     Arc::clone(&environment_manager),
+                    self.thread_manager.mcp_manager(),
                 )
                 .await;
                 remote_plugin_detail_to_info(remote_detail, app_summaries)
@@ -1611,10 +1613,11 @@ impl PluginRequestProcessor {
         let environment_manager = self.thread_manager.environment_manager();
         let (all_connectors_result, accessible_connectors_result) = tokio::join!(
             connectors::list_all_connectors_with_options(config, /*force_refetch*/ false),
-            connectors::list_accessible_connectors_from_mcp_tools_with_environment_manager(
+            connectors::list_accessible_connectors_from_mcp_tools_with_mcp_manager(
                 config,
                 /*force_refetch*/ true,
-                Arc::clone(&environment_manager)
+                Arc::clone(&environment_manager),
+                self.thread_manager.mcp_manager(),
             ),
         );
 
@@ -1881,6 +1884,7 @@ async fn load_plugin_app_summaries(
     config: &Config,
     plugin_apps: &[codex_plugin::AppConnectorId],
     environment_manager: Arc<EnvironmentManager>,
+    mcp_manager: Arc<McpManager>,
 ) -> Vec<AppSummary> {
     if plugin_apps.is_empty() {
         return Vec::new();
@@ -1900,10 +1904,11 @@ async fn load_plugin_app_summaries(
     let plugin_connectors = connectors::connectors_for_plugin_apps(connectors, plugin_apps);
 
     let accessible_connectors =
-        match connectors::list_accessible_connectors_from_mcp_tools_with_environment_manager(
+        match connectors::list_accessible_connectors_from_mcp_tools_with_mcp_manager(
             config,
             /*force_refetch*/ false,
             environment_manager,
+            mcp_manager,
         )
         .await
         {