Test selected capabilities across availability and resume (#30157)

## Why

This stack crosses World State, executor skills, selected plugin
metadata, MCP processes, connectors, dynamic environments, and resume.
This PR adds two end-to-end scenarios that validate those pieces
together.

Both tests enable `deferred_executor`, so they exercise the real
delayed-environment path.

## Scenario 1: availability across turns and resume

```text
1. Start a thread with one selected plugin root bound to E1.
2. E1 is unavailable.
   - executor skill is absent
   - selected MCP is absent
   - connector has no selected-plugin attribution
3. Start E1 and register the same stable environment ID.
4. Start a new turn.
   - the executor skill appears through World State
   - its body beats a colliding host skill
   - the selected MCP tool is advertised and executes inside E1
   - the connector is attributed to the selected plugin
5. Start another turn without changing E1.
   - the MCP PID stays the same, proving runtime reuse
6. Restart app-server and resume the thread.
   - durable selected-root intent is restored
   - skills, MCP, and connector attribution are restored
   - a new MCP PID proves ephemeral process state was rebuilt
```

## Scenario 2: availability changes inside one turn

```text
1. Start a turn while E1 is unavailable.
2. The first model sample sees no executor skill, MCP, or selected connector.
3. The turn pauses on request_user_input.
4. Start E1 and register it while that same turn is still active.
5. Continue the turn.
6. The very next model sample sees:
   - the executor skill catalog
   - the selected MCP tool
   - selected-plugin connector attribution
7. The model calls the MCP, and its output proves execution happened inside E1.
```

This second scenario specifically protects the aeon-style behavior:
capability state is captured again for every sampling step, not only at
the next user turn.

## Scope

These are integration tests only. They do not add a combinatorial matrix
for unsupported plugin-file mutation, environment generations, transport
disconnects, or delayed `required = true` executor MCPs.

Author: jif-oai

codex-rs/app-server/tests/suite/v2/app_list.rs

@@ -1533,7 +1533,7 @@ impl ServerHandler for AppListMcpServer {
     }
 }
 
-async fn start_apps_server_with_delays(
+pub(super) async fn start_apps_server_with_delays(
     connectors: Vec<AppInfo>,
     tools: Vec<Tool>,
     directory_delay: Duration,
@@ -1693,7 +1693,7 @@ async fn list_directory_connectors(
     }
 }
 
-fn connector_tool(connector_id: &str, connector_name: &str) -> Result<Tool> {
+pub(super) fn connector_tool(connector_id: &str, connector_name: &str) -> Result<Tool> {
     let schema: JsonObject = serde_json::from_value(json!({
         "type": "object",
         "additionalProperties": false

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -56,6 +56,8 @@ mod request_user_input;
 mod request_validation;
 mod review;
 mod safety_check_downgrade;
+#[cfg(not(target_os = "windows"))]
+mod selected_capability_stack;
 mod skills_list;
 mod sleep;
 mod thread_archive;