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/test/unit/feishu-client.test.ts

@@ -2,7 +2,7 @@ import { describe, expect, it, vi } from "vitest";
 import { mkdtemp, rm, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { sendFile, sendTextMessage, type FeishuRuntime } from "../../src/feishu/client.js";
+import { sendFile, sendLongText, sendTextMessage, type FeishuRuntime } from "../../src/feishu/client.js";
 
 describe("Feishu client helpers", () => {
   it("accepts top-level file_key from the Lark SDK file upload response", async () => {
@@ -32,6 +32,21 @@ describe("Feishu client helpers", () => {
 
     await expect(sendTextMessage(rt, "chat-1", "hello")).resolves.toBe("message-1");
   });
+
+  it("sends long text in chunks and returns the last successful message id", async () => {
+    let messageNumber = 0;
+    const messageCreate = vi.fn(async () => ({ data: { message_id: `message-${++messageNumber}` } }));
+    const rt = mockRuntime({ fileCreate: vi.fn(), messageCreate });
+
+    await expect(sendLongText(rt, "chat-1", "x".repeat(16_005))).resolves.toBe("message-3");
+
+    expect(messageCreate).toHaveBeenCalledTimes(3);
+    const sentTexts = messageCreate.mock.calls.map(([payload]) => {
+      const content = (payload as { data: { content: string } }).data.content;
+      return (JSON.parse(content) as { text: string }).text;
+    });
+    expect(sentTexts.map((text) => text.length)).toEqual([8000, 8000, 5]);
+  });
 });
 
 function mockRuntime(options: {