Parse first order skill/connector mentions (#11547)

This PR introduces a skill-expansion mechanism for mentions so nested or
skill or connection mentions are expanded if present in skills invoked
by the user. This keeps behavior aligned with existing mention handling
while extending coverage to deeper scenarios. With these changes, users
can create skills that invoke connectors, and skills that invoke other
skills.

Replaces #10863, which is not needed with the addition of
[search_tool_bm25](https://github.com/openai/codex/issues/10657)

codex-rs/core/src/tasks/mod.rs

@@ -121,6 +121,7 @@ impl Session {
     ) {
         self.abort_all_tasks(TurnAbortReason::Replaced).await;
         self.clear_mcp_tool_selection().await;
+        self.clear_connector_selection().await;
         self.seed_initial_context_if_needed(turn_context.as_ref())
             .await;