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/apps_processor.rs

@@ -88,11 +88,19 @@ impl AppsRequestProcessor {
         let request = request_id.clone();
         let outgoing = Arc::clone(&self.outgoing);
         let environment_manager = self.thread_manager.environment_manager();
+        let mcp_manager = self.thread_manager.mcp_manager();
         let shutdown_token = self.shutdown_token.child_token();
         tokio::spawn(async move {
             tokio::select! {
                 _ = shutdown_token.cancelled() => {}
-                _ = Self::apps_list_task(outgoing, request, params, config, environment_manager) => {}
+                _ = Self::apps_list_task(
+                    outgoing,
+                    request,
+                    params,
+                    config,
+                    environment_manager,
+                    mcp_manager,
+                ) => {}
             }
         });
         Ok(None)
@@ -108,11 +116,15 @@ impl AppsRequestProcessor {
         params: AppsListParams,
         config: Config,
         environment_manager: Arc<EnvironmentManager>,
+        mcp_manager: Arc<McpManager>,
     ) {
         let retry_params = params.clone();
         let retry_config = config.clone();
         let retry_environment_manager = Arc::clone(&environment_manager);
-        let result = Self::apps_list_response(&outgoing, params, config, environment_manager).await;
+        let retry_mcp_manager = Arc::clone(&mcp_manager);
+        let result =
+            Self::apps_list_response(&outgoing, params, config, environment_manager, mcp_manager)
+                .await;
         let should_retry = result
             .as_ref()
             .is_ok_and(|(_, codex_apps_ready)| !codex_apps_ready);
@@ -128,6 +140,7 @@ impl AppsRequestProcessor {
                 retry_params,
                 retry_config,
                 retry_environment_manager,
+                retry_mcp_manager,
             )
             .await
             {
@@ -141,6 +154,7 @@ impl AppsRequestProcessor {
         params: AppsListParams,
         config: Config,
         environment_manager: Arc<EnvironmentManager>,
+        mcp_manager: Arc<McpManager>,
     ) -> Result<(AppsListResponse, bool), JSONRPCErrorError> {
         let AppsListParams {
             cursor,
@@ -167,14 +181,14 @@ impl AppsRequestProcessor {
         let accessible_config = config.clone();
         let accessible_tx = tx.clone();
         tokio::spawn(async move {
-            let result =
-                connectors::list_accessible_connectors_from_mcp_tools_with_environment_manager(
-                    &accessible_config,
-                    force_refetch,
-                    Arc::clone(&environment_manager),
-                )
-                .await
-                .map_err(|err| format!("failed to load accessible apps: {err}"));
+            let result = connectors::list_accessible_connectors_from_mcp_tools_with_mcp_manager(
+                &accessible_config,
+                force_refetch,
+                Arc::clone(&environment_manager),
+                mcp_manager,
+            )
+            .await
+            .map_err(|err| format!("failed to load accessible apps: {err}"));
             let _ = accessible_tx.send(AppListLoadResult::Accessible(result));
         });