fix: introduce EventMsg::TurnAborted (#2365)

Introduces `EventMsg::TurnAborted` that should be sent in response to
`Op::Interrupt`.

In the MCP server, updates the handling of a
`ClientRequest::InterruptConversation` request such that it sends the
`Op::Interrupt` but does not respond to the request until it sees an
`EventMsg::TurnAborted`.

codex-rs/tui/src/chatwidget.rs

@@ -635,6 +635,7 @@ impl ChatWidget<'_> {
             EventMsg::TaskComplete(TaskCompleteEvent { .. }) => self.on_task_complete(),
             EventMsg::TokenCount(token_usage) => self.on_token_count(token_usage),
             EventMsg::Error(ErrorEvent { message }) => self.on_error(message),
+            EventMsg::TurnAborted(_) => self.on_error("Turn interrupted".to_owned()),
             EventMsg::PlanUpdate(update) => self.on_plan_update(update),
             EventMsg::ExecApprovalRequest(ev) => self.on_exec_approval_request(id, ev),
             EventMsg::ApplyPatchApprovalRequest(ev) => self.on_apply_patch_approval_request(id, ev),