skills: make backend plugin skills invocable without an executor (#27387)

## Why

#27198 made the extension-owned `codex_apps` MCP connection the hosted
plugin runtime, but its `mcp/skill` resources still bypassed the skills
extension. App-server could list and read those resources through
generic MCP APIs, but a thread with no selected environment did not
expose them in the model's skills catalog or load their `SKILL.md`
through `$skill`.

Hosted skills should stay remote while using the same typed catalog,
source authority, deduplication, bounded contextual catalog, and
selected-skill prompt injection as host and executor skills. They should
not be downloaded or exposed as ambient filesystem paths.

## What changed

- Add a session-scoped `McpResourceClient` over the replaceable MCP
connection manager so resource list/read calls follow startup and
refresh replacements.
- Add a `BackendSkillProvider` that pages `codex_apps` resources,
accepts bounded and validated `mcp/skill` entries, and reads a selected
skill's `SKILL.md` through the same MCP connection.
- Register the remote provider in app-server and include it in the
skills catalog even when a thread has no selected capability roots or
executor.
- Contribute hosted skill metadata through the bounded
`AvailableSkillsInstructions` developer-context path, exclude remote
entries from per-turn catalog injection, and classify `<skills>`
messages as contextual developer content so rollback can trim and
rebuild them correctly.

## Testing

- Extend the app-server MCP resource integration test with
`environments: []` to exercise two-page discovery, filter a
non-`mcp/skill` resource, verify the escaped developer catalog entry and
user-role `<skill>` fragment containing the fetched `SKILL.md`, and
preserve generic MCP resource reads.
- Add core event-mapping coverage that classifies `<skills>` developer
messages as contextual history.

codex-rs/core/src/session/tests.rs

@@ -4940,13 +4940,13 @@ pub(crate) async fn make_session_and_context() -> (Session, TurnContext) {
     );
 
     let services = SessionServices {
-        mcp_connection_manager: arc_swap::ArcSwap::from_pointee(
+        mcp_connection_manager: Arc::new(arc_swap::ArcSwap::from_pointee(
             McpConnectionManager::new_uninitialized_with_permission_profile(
                 &config.permissions.approval_policy,
                 config.permissions.permission_profile(),
                 config.prefix_mcp_tool_names(),
             ),
-        ),
+        )),
         mcp_startup_cancellation_token: Mutex::new(CancellationToken::new()),
         unified_exec_manager: UnifiedExecProcessManager::new(
             config.background_terminal_max_timeout,
@@ -7018,13 +7018,13 @@ where
     );
 
     let services = SessionServices {
-        mcp_connection_manager: arc_swap::ArcSwap::from_pointee(
+        mcp_connection_manager: Arc::new(arc_swap::ArcSwap::from_pointee(
             McpConnectionManager::new_uninitialized_with_permission_profile(
                 &config.permissions.approval_policy,
                 config.permissions.permission_profile(),
                 config.prefix_mcp_tool_names(),
             ),
-        ),
+        )),
         mcp_startup_cancellation_token: Mutex::new(CancellationToken::new()),
         unified_exec_manager: UnifiedExecProcessManager::new(
             config.background_terminal_max_timeout,