feat: use Telegram native Rich Message for assistant replies

Co-authored-by: MuziIsabel <MuziIsabel@users.noreply.github.com>

tests/rich-format.test.ts

@@ -0,0 +1,31 @@
+import assert from 'node:assert/strict'
+import test from 'node:test'
+import {
+  RICH_MESSAGE_MAX_CHARS,
+  splitRichMarkdown,
+} from '../src/rich-format.ts'
+
+test('splitRichMarkdown keeps small markdown as one chunk', () => {
+  const markdown = '# 标题\n\n- a\n- b\n\n```js\nx\n```\n'
+  assert.deepEqual(splitRichMarkdown(markdown), [markdown])
+})
+
+test('splitRichMarkdown splits long plain text without losing content', () => {
+  const markdown = 'a'.repeat(RICH_MESSAGE_MAX_CHARS + 100)
+  const chunks = splitRichMarkdown(markdown)
+  assert.ok(chunks.length > 1)
+  assert.equal(chunks.join(''), markdown)
+})
+
+test('splitRichMarkdown keeps long fenced code blocks balanced', () => {
+  const opening = '```js\n'
+  const closing = '\n```'
+  const markdown = `${opening}${'x'.repeat(RICH_MESSAGE_MAX_CHARS + 1000)}${closing}`
+  const chunks = splitRichMarkdown(markdown)
+  assert.ok(chunks.length > 1)
+  for (const chunk of chunks) {
+    assert.ok(chunk.trimStart().startsWith('```'), 'chunk should start with a fence')
+    assert.ok(chunk.trimEnd().endsWith('```'), 'chunk should end with a fence')
+  }
+  assert.equal(chunks.join('').replace(/```js\n/g, '').replace(/\n```/g, ''), 'x'.repeat(RICH_MESSAGE_MAX_CHARS + 1000))
+})