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/Cargo.lock

@@ -1982,6 +1982,7 @@ dependencies = [
  "codex-image-generation-extension",
  "codex-login",
  "codex-mcp",
+ "codex-mcp-extension",
  "codex-memories-extension",
  "codex-memories-write",
  "codex-model-provider",
@@ -2920,6 +2921,7 @@ name = "codex-extension-api"
 version = "0.0.0"
 dependencies = [
  "async-trait",
+ "codex-config",
  "codex-context-fragments",
  "codex-protocol",
  "codex-tools",
@@ -3247,6 +3249,23 @@ dependencies = [
  "url",
 ]
 
+[[package]]
+name = "codex-mcp-extension"
+version = "0.0.0"
+dependencies = [
+ "async-trait",
+ "codex-config",
+ "codex-core",
+ "codex-core-plugins",
+ "codex-extension-api",
+ "codex-features",
+ "codex-login",
+ "codex-mcp",
+ "pretty_assertions",
+ "tempfile",
+ "tokio",
+]
+
 [[package]]
 name = "codex-mcp-server"
 version = "0.0.0"