feat(threads): paginate full history via summaries endpoint (#1022)

* feat(threads): add paginated summaries API and load full history

* fix(threads): address summaries review feedback

- validate summaries sort params and log gateway failures
- page frontend thread summaries without stale query keys or silent truncation
- export router modules and tighten thread list typing

Refs: 2901804166, 2901804176, 2901804179, 2901804180, 2901804183, 2901804187, 2901804191

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/core/threads/utils.ts

@@ -1,6 +1,5 @@
 import type { Message } from "@langchain/langgraph-sdk";
 
-import type { AgentThread } from "./types";
 
 export function pathOfThread(threadId: string) {
   return `/workspace/chats/${threadId}`;
@@ -19,6 +18,6 @@ export function textOfMessage(message: Message) {
   return null;
 }
 
-export function titleOfThread(thread: AgentThread) {
+export function titleOfThread(thread: { values?: { title?: string | null } | null }) {
   return thread.values?.title ?? "Untitled";
 }