Activate selected executor plugin MCPs in app-server (#27893)

## Why

#27870 teaches the MCP extension how to discover stdio MCP servers
declared by a selected executor plugin, but app-server does not yet
install that contributor or initialize its per-thread state. As a
result, `thread/start.selectedCapabilityRoots` can select the plugin
while its MCP servers remain inactive.

This PR closes that app-server wiring gap:

```text
thread/start(selectedCapabilityRoots)
    -> initialize the thread's selected-plugin MCP snapshot
    -> read the selected plugin's .mcp.json through its environment
    -> start declared stdio servers in that environment
    -> expose their tools only on the selected thread
```

## What changed

- Install the selected-executor-plugin MCP contributor in app-server
using the existing shared `EnvironmentManager`.
- Initialize its frozen thread snapshot when `thread/start` includes
selected capability roots.
- Document that selected plugin stdio MCPs are activated in their owning
environment.
- Add an app-server E2E covering the complete selection-to-tool-call
path.

The E2E verifies that:

- the selected MCP process receives an executor-only environment value,
proving the tool runs through the selected environment;
- the MCP tool is advertised to the model and can be called;
- a normal MCP config reload does not discard the thread's frozen
selected-plugin registration;
- another thread without the selected root does not see the MCP server.

## Scope

- Existing sessions without `selectedCapabilityRoots` are unchanged.
- Only stdio MCP declarations are activated. HTTP declarations remain
inactive.
- This does not change selected-root persistence across resume/fork or
add hosted-plugin behavior.

## Verification

- Focused app-server E2E:
`selected_executor_plugin_exposes_its_stdio_mcp_only_to_that_thread`

## Stack

Stacked on #27870.

codex-rs/app-server/src/message_processor.rs

@@ -329,7 +329,7 @@ impl MessageProcessor {
         let restriction_product = session_source.restriction_product();
         let executor_skill_provider: Arc<dyn codex_skills_extension::SkillProvider> = Arc::new(
             codex_skills_extension::ExecutorSkillProvider::new_with_restriction_product(
-                environment_manager_for_extensions,
+                Arc::clone(&environment_manager_for_extensions),
                 restriction_product,
             ),
         );
@@ -352,6 +352,7 @@ impl MessageProcessor {
                         analytics_events_client: analytics_events_client.clone(),
                         thread_manager: thread_manager.clone(),
                         goal_service: Arc::clone(&goal_service),
+                        environment_manager: Arc::clone(&environment_manager_for_extensions),
                         executor_skill_provider: Arc::clone(&executor_skill_provider),
                         thread_store: Arc::clone(&thread_store),
                     },