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.ts

@@ -0,0 +1,61 @@
+/**
+ * In-memory cache of Slack threads the bot has participated in.
+ * Used to auto-respond in threads without requiring @mention after the first reply.
+ * Follows a similar TTL pattern to the MS Teams and Telegram sent-message caches.
+ */
+
+const TTL_MS = 24 * 60 * 60 * 1000; // 24 hours
+const MAX_ENTRIES = 5000;
+
+const threadParticipation = new Map<string, number>();
+
+function makeKey(accountId: string, channelId: string, threadTs: string): string {
+  return `${accountId}:${channelId}:${threadTs}`;
+}
+
+function evictExpired(): void {
+  const now = Date.now();
+  for (const [key, timestamp] of threadParticipation) {
+    if (now - timestamp > TTL_MS) {
+      threadParticipation.delete(key);
+    }
+  }
+}
+
+export function recordSlackThreadParticipation(
+  accountId: string,
+  channelId: string,
+  threadTs: string,
+): void {
+  if (!accountId || !channelId || !threadTs) {
+    return;
+  }
+  if (threadParticipation.size >= MAX_ENTRIES) {
+    evictExpired();
+  }
+  threadParticipation.set(makeKey(accountId, channelId, threadTs), Date.now());
+}
+
+export function hasSlackThreadParticipation(
+  accountId: string,
+  channelId: string,
+  threadTs: string,
+): boolean {
+  if (!accountId || !channelId || !threadTs) {
+    return false;
+  }
+  const key = makeKey(accountId, channelId, threadTs);
+  const timestamp = threadParticipation.get(key);
+  if (timestamp == null) {
+    return false;
+  }
+  if (Date.now() - timestamp > TTL_MS) {
+    threadParticipation.delete(key);
+    return false;
+  }
+  return true;
+}
+
+export function clearSlackThreadParticipationCache(): void {
+  threadParticipation.clear();
+}