feat: improve outbound message chunking with code-block-aware splitting

- Increase max message length from 4000 to 8000 chars
- Add splitAtBoundary: paragraph > newline > space priority, never split
  inside fenced code blocks (move split to before opening fence)
- Add sendLongText: sends multi-chunk long messages sequentially
- Update PatchableTextStream.flush to use splitAtBoundary
- Add unit tests (6) for splitting edge cases

hub/src/feishu/client.ts

@@ -9,6 +9,7 @@ import * as lark from "@larksuiteoapi/node-sdk";
 import { readFile, writeFile, mkdir } from "node:fs/promises";
 import { dirname } from "node:path";
 import type { FastifyBaseLogger } from "fastify";
+import { DEFAULT_MAX_MESSAGE_LENGTH, splitAtBoundary } from "./textStream.js";
 
 export interface FeishuConfig {
   readonly appId: string;
@@ -157,6 +158,18 @@ export async function sendTextMessage(rt: FeishuRuntime, chatId: string, text: s
   }
 }
 
+/** Send long text as multiple Feishu messages. Returns the last successful message_id. */
+export async function sendLongText(rt: FeishuRuntime, chatId: string, text: string): Promise<string | null> {
+  let lastMessageId: string | null = null;
+  for (const chunk of splitAtBoundary(text, DEFAULT_MAX_MESSAGE_LENGTH)) {
+    const messageId = await sendTextMessage(rt, chatId, chunk);
+    if (messageId !== null) {
+      lastMessageId = messageId;
+    }
+  }
+  return lastMessageId;
+}
+
 /** Send a plain text message (fire-and-forget). */
 export async function sendText(rt: FeishuRuntime, chatId: string, text: string): Promise<void> {
   await sendTextMessage(rt, chatId, text);