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: {

hub/test/unit/text-split.test.ts

@@ -0,0 +1,43 @@
+import { describe, expect, it } from "vitest";
+import { splitAtBoundary } from "../../src/feishu/textStream.js";
+
+describe("splitAtBoundary", () => {
+  it("keeps short text in a single chunk", () => {
+    expect(splitAtBoundary("short text", 20)).toEqual(["short text"]);
+  });
+
+  it("keeps text at exactly maxLength in a single chunk", () => {
+    const text = "x".repeat(20);
+
+    expect(splitAtBoundary(text, 20)).toEqual([text]);
+  });
+
+  it("splits text exceeding maxLength into chunks at or below maxLength", () => {
+    const chunks = splitAtBoundary("x".repeat(45), 20);
+
+    expect(chunks).toHaveLength(3);
+    expect(chunks.every((chunk) => chunk.length <= 20)).toBe(true);
+    expect(chunks.join("")).toBe("x".repeat(45));
+  });
+
+  it("moves a split before a fenced code block when the boundary lands inside it", () => {
+    const prefix = "Before code starts\n";
+    const codeBlock = "```ts\nx\n```\n";
+    const text = `${prefix}${codeBlock}After`;
+    const chunks = splitAtBoundary(text, prefix.length + 4);
+
+    expect(chunks).toEqual([prefix, `${codeBlock}After`]);
+  });
+
+  it("prefers paragraph boundaries over newline boundaries", () => {
+    const text = "para one\n\nline two\nline three";
+
+    expect(splitAtBoundary(text, 22)).toEqual(["para one\n\n", "line two\nline three"]);
+  });
+
+  it("prefers newline boundaries over space boundaries", () => {
+    const text = "alpha beta\ngamma delta epsilon";
+
+    expect(splitAtBoundary(text, 24)).toEqual(["alpha beta\n", "gamma delta epsilon"]);
+  });
+});