fix: add session cursor for tool output

src/shared/session-cursor.test.ts

@@ -0,0 +1,53 @@
+import { beforeEach, describe, expect, it } from "bun:test"
+import { getNewMessages, resetMessageCursor } from "./session-cursor"
+
+describe("getNewMessages", () => {
+  const sessionID = "session-123"
+
+  const buildMessage = (id: string, created: number) => ({
+    info: { id, time: { created } },
+  })
+
+  beforeEach(() => {
+    resetMessageCursor(sessionID)
+  })
+
+  it("returns all messages on first read and none on repeat", () => {
+    // #given
+    const messages = [buildMessage("m1", 1), buildMessage("m2", 2)]
+
+    // #when
+    const first = getNewMessages(sessionID, messages)
+    const second = getNewMessages(sessionID, messages)
+
+    // #then
+    expect(first).toEqual(messages)
+    expect(second).toEqual([])
+  })
+
+  it("returns only new messages after cursor advances", () => {
+    // #given
+    const messages = [buildMessage("m1", 1), buildMessage("m2", 2)]
+    getNewMessages(sessionID, messages)
+    const extended = [...messages, buildMessage("m3", 3)]
+
+    // #when
+    const next = getNewMessages(sessionID, extended)
+
+    // #then
+    expect(next).toEqual([extended[2]])
+  })
+
+  it("resets when message history shrinks", () => {
+    // #given
+    const messages = [buildMessage("m1", 1), buildMessage("m2", 2)]
+    getNewMessages(sessionID, messages)
+    const shorter = [buildMessage("n1", 1)]
+
+    // #when
+    const next = getNewMessages(sessionID, shorter)
+
+    // #then
+    expect(next).toEqual(shorter)
+  })
+})