fix(tdlib): handle 1.8.64 renames in searchChatMessages + message reply_to

Audit of every TDLib call site against the live 1.8.64 schema in
node_modules/@prebuilt-tdlib/types/tdlib-types.d.ts surfaced three
additional silent breakages beyond the getForumTopics fix in 106700b.

1. searchChatMessages parameter restructure
   The top-level `message_thread_id` and `saved_messages_topic_id`
   request fields were collapsed into a single tagged-union
   `topic_id: MessageTopic$Input`. Three call sites affected:

   - topics.ts getTopicMessages — was passing message_thread_id, now
     sends topic_id with the messageTopicForum variant carrying
     forum_topic_id. Without this the topic scan returns the whole
     channel (or nothing) instead of just the topic.
   - download.ts getChannelMessages — used to pass message_thread_id: 0;
     just omit the topic_id field entirely for a flat scan.
   - rebuild.ts — same treatment.

2. message.reply_to_message_id replaced with reply_to tagged union
   On incoming messages, the flat `reply_to_message_id` field was
   replaced with `reply_to: MessageReplyTo` (messageReplyToMessage or
   messageReplyToStory). Our reply-chain grouping needs the message-ID
   case.

   Added extractReplyToMessageId() that reads both old and new shapes
   so a transition build or future downgrade still works.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

worker/src/tdlib/topics.ts

@@ -239,14 +239,20 @@ export async function getTopicMessages(
     }>(client, {
       _: "searchChatMessages",
       chat_id: Number(chatId),
+      // TDLib 1.8.64+ replaced the top-level `message_thread_id` and
+      // `saved_messages_topic_id` parameters with a single tagged-union
+      // `topic_id: MessageTopic$Input`. For a forum topic, use the
+      // messageTopicForum variant carrying the forum_topic_id.
+      topic_id: {
+        _: "messageTopicForum",
+        forum_topic_id: Number(topicId),
+      },
       query: "",
-      message_thread_id: Number(topicId),
       from_message_id: currentFromId,
       offset: 0,
       limit: Math.min(limit, 100),
       filter: null,
       sender_id: null,
-      saved_messages_topic_id: 0,
     });
 
     if (!result.messages || result.messages.length === 0) break;