fix(frontend): deduplicate restored thread messages (#2958)

* fix(frontend): fix duplicate messages when reopening agent sessions (#2957)

* make format

* fix(frontend): retry pending thread history loads

frontend/tests/unit/core/threads/message-merge.test.ts

@@ -0,0 +1,64 @@
+import type { Message } from "@langchain/langgraph-sdk";
+import { expect, test } from "vitest";
+
+import { mergeMessages } from "@/core/threads/hooks";
+
+test("mergeMessages removes duplicate messages already present in history", () => {
+  const human = {
+    id: "human-1",
+    type: "human",
+    content: "Design an agent",
+  } as Message;
+  const ai = {
+    id: "ai-1",
+    type: "ai",
+    content: "Let's design it.",
+  } as Message;
+
+  expect(mergeMessages([human, ai, human, ai], [], [])).toEqual([human, ai]);
+});
+
+test("mergeMessages lets live thread messages replace overlapping history", () => {
+  const oldHuman = {
+    id: "human-1",
+    type: "human",
+    content: "old",
+  } as Message;
+  const liveHuman = {
+    id: "human-1",
+    type: "human",
+    content: "live",
+  } as Message;
+  const oldAi = {
+    id: "ai-1",
+    type: "ai",
+    content: "old",
+  } as Message;
+  const liveAi = {
+    id: "ai-1",
+    type: "ai",
+    content: "live",
+  } as Message;
+
+  expect(mergeMessages([oldHuman, oldAi], [liveHuman, liveAi], [])).toEqual([
+    liveHuman,
+    liveAi,
+  ]);
+});
+
+test("mergeMessages deduplicates tool messages by tool_call_id", () => {
+  const oldTool = {
+    id: "tool-message-old",
+    type: "tool",
+    tool_call_id: "call-1",
+    content: "old",
+  } as Message;
+  const liveTool = {
+    id: "tool-message-live",
+    type: "tool",
+    tool_call_id: "call-1",
+    content: "live",
+  } as Message;
+
+  expect(mergeMessages([oldTool], [liveTool], [])).toEqual([liveTool]);
+});