feat(slack): track thread participation for auto-reply without @mention (#29165)

* feat(slack): track thread participation for auto-reply without @mention

* fix(slack): scope thread participation cache by accountId and capture actual reply thread ts

* fix(slack): capture reply thread ts from all delivery paths and only after success

* Slack: add changelog for thread participation cache behavior

---------

Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

src/slack/sent-thread-cache.test.ts

@@ -0,0 +1,58 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import {
+  clearSlackThreadParticipationCache,
+  hasSlackThreadParticipation,
+  recordSlackThreadParticipation,
+} from "./sent-thread-cache.js";
+
+describe("slack sent-thread-cache", () => {
+  afterEach(() => {
+    clearSlackThreadParticipationCache();
+    vi.restoreAllMocks();
+  });
+
+  it("records and checks thread participation", () => {
+    recordSlackThreadParticipation("A1", "C123", "1700000000.000001");
+    expect(hasSlackThreadParticipation("A1", "C123", "1700000000.000001")).toBe(true);
+  });
+
+  it("returns false for unrecorded threads", () => {
+    expect(hasSlackThreadParticipation("A1", "C123", "1700000000.000001")).toBe(false);
+  });
+
+  it("distinguishes different channels and threads", () => {
+    recordSlackThreadParticipation("A1", "C123", "1700000000.000001");
+    expect(hasSlackThreadParticipation("A1", "C123", "1700000000.000002")).toBe(false);
+    expect(hasSlackThreadParticipation("A1", "C456", "1700000000.000001")).toBe(false);
+  });
+
+  it("scopes participation by accountId", () => {
+    recordSlackThreadParticipation("A1", "C123", "1700000000.000001");
+    expect(hasSlackThreadParticipation("A2", "C123", "1700000000.000001")).toBe(false);
+    expect(hasSlackThreadParticipation("A1", "C123", "1700000000.000001")).toBe(true);
+  });
+
+  it("ignores empty accountId, channelId, or threadTs", () => {
+    recordSlackThreadParticipation("", "C123", "1700000000.000001");
+    recordSlackThreadParticipation("A1", "", "1700000000.000001");
+    recordSlackThreadParticipation("A1", "C123", "");
+    expect(hasSlackThreadParticipation("", "C123", "1700000000.000001")).toBe(false);
+    expect(hasSlackThreadParticipation("A1", "", "1700000000.000001")).toBe(false);
+    expect(hasSlackThreadParticipation("A1", "C123", "")).toBe(false);
+  });
+
+  it("clears all entries", () => {
+    recordSlackThreadParticipation("A1", "C123", "1700000000.000001");
+    recordSlackThreadParticipation("A1", "C456", "1700000000.000002");
+    clearSlackThreadParticipationCache();
+    expect(hasSlackThreadParticipation("A1", "C123", "1700000000.000001")).toBe(false);
+    expect(hasSlackThreadParticipation("A1", "C456", "1700000000.000002")).toBe(false);
+  });
+
+  it("expired entries return false and are cleaned up on read", () => {
+    recordSlackThreadParticipation("A1", "C123", "1700000000.000001");
+    // Advance time past the 24-hour TTL
+    vi.spyOn(Date, "now").mockReturnValue(Date.now() + 25 * 60 * 60 * 1000);
+    expect(hasSlackThreadParticipation("A1", "C123", "1700000000.000001")).toBe(false);
+  });
+});