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/request_processors/thread_processor.rs

@@ -1096,6 +1096,7 @@ impl ThreadRequestProcessor {
         let mut thread_extension_init = ExtensionDataInit::new();
         if !selected_capability_roots.is_empty() {
             thread_extension_init.insert(selected_capability_roots);
+            codex_mcp_extension::initialize_executor_plugin_thread_data(&mut thread_extension_init);
         }
         let create_thread_started_at = std::time::Instant::now();
         let NewThread {