test: add vitest unit coverage

src/client/src/chatHistoryCache.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from "vitest";
+import { mergeChatHistory, type RawMessagePage } from "./chatHistoryCache";
+
+function page(start: number, total: number, messages: string[]): RawMessagePage {
+  return { start, total, messages };
+}
+
+describe("mergeChatHistory", () => {
+  it("merges adjacent cached and incoming pages", () => {
+    const merged = mergeChatHistory(page(2, 5, ["c", "d", "e"]), page(0, 5, ["a", "b"]));
+
+    expect(merged).toEqual(page(0, 5, ["a", "b", "c", "d", "e"]));
+  });
+
+  it("uses incoming history when totals changed", () => {
+    const incoming = page(0, 2, ["fresh-a", "fresh-b"]);
+
+    expect(mergeChatHistory(page(0, 1, ["stale"]), incoming)).toEqual(incoming);
+  });
+});