fix(telegram): move fenced-code-block splitting into Telegram-specific helper

Move the fenced-code-block-aware splitting logic out of the shared
split_message helper (used by Signal, Slack, Discord, Weixin, etc.)
and into a Telegram-specific _split_telegram_markdown function.

The shared split_message remains a plain-text chunker. The Telegram
channel now uses _split_telegram_markdown for its raw Markdown paths
that feed _markdown_to_telegram_html, preventing broken HTML rendering
when splits fall inside fenced code blocks.

Also fixes a regression where content beginning with whitespace before
a fence could emit a whitespace-only chunk.

Addresses review feedback on #4257.

tests/channels/test_telegram_channel.py

@@ -17,6 +17,8 @@ from nanobot.channels.telegram import (
     TELEGRAM_REPLY_CONTEXT_MAX_LEN,
     TelegramChannel,
     TelegramConfig,
+    _markdown_to_telegram_html,
+    _split_telegram_markdown,
     _StreamBuf,
 )
 
@@ -179,6 +181,67 @@ def _make_telegram_update(
     return SimpleNamespace(message=message, effective_user=user)
 
 
+def _assert_code_blocks_render_balanced(chunks: list[str]) -> None:
+    for chunk in chunks:
+        html = _markdown_to_telegram_html(chunk)
+        assert html.count("<pre><code>") == html.count("</code></pre>")
+
+
+def test_split_telegram_markdown_inside_code_block_moves_before_fence() -> None:
+    content = "Intro paragraph.\n```python\nprint('a')\nprint('b')\n```\nDone"
+
+    chunks = _split_telegram_markdown(content, max_len=35)
+
+    assert chunks[0] == "Intro paragraph.\n"
+    assert chunks[1].startswith("```python\nprint('a')")
+    _assert_code_blocks_render_balanced(chunks)
+
+
+def test_split_telegram_markdown_long_code_block_closes_and_reopens() -> None:
+    content = "```python\n" + ("print('line one')\n" * 6) + "```\nDone"
+
+    chunks = _split_telegram_markdown(content, max_len=60)
+
+    assert len(chunks) > 1
+    assert all(len(chunk) <= 60 for chunk in chunks)
+    assert chunks[0].startswith("```python\n")
+    assert chunks[0].endswith("\n```")
+    assert chunks[1].startswith("```python\n")
+    _assert_code_blocks_render_balanced(chunks)
+
+
+def test_split_telegram_markdown_multiple_code_blocks() -> None:
+    content = (
+        "First\n"
+        "```js\n"
+        "one();\n"
+        "```\n"
+        "Middle paragraph here\n"
+        "```py\n"
+        "two()\n"
+        "three()\n"
+        "```\n"
+        "End"
+    )
+
+    chunks = _split_telegram_markdown(content, max_len=55)
+
+    assert chunks[0].endswith("Middle paragraph here\n")
+    assert chunks[1].startswith("```py\n")
+    _assert_code_blocks_render_balanced(chunks)
+
+
+def test_split_telegram_markdown_leading_whitespace_before_fence() -> None:
+    content = "\n```python\n" + ("print('line one')\n" * 6) + "```\nDone"
+
+    chunks = _split_telegram_markdown(content, max_len=60)
+
+    assert chunks
+    assert all(chunk.strip() for chunk in chunks)
+    assert chunks[0].startswith("```python\n")
+    _assert_code_blocks_render_balanced(chunks)
+
+
 @pytest.mark.asyncio
 async def test_start_creates_separate_pools_with_proxy(monkeypatch) -> None:
     _FakeHTTPXRequest.clear()

tests/utils/test_helpers.py

@@ -5,56 +5,3 @@ def test_split_message_no_code_blocks_unchanged():
     content = "alpha beta gamma delta"
 
     assert split_message(content, max_len=12) == ["alpha beta", "gamma delta"]
-
-
-def test_split_message_outside_code_block_unchanged():
-    content = "alpha beta gamma delta\n```python\nx = 1\n```\ndone"
-
-    chunks = split_message(content, max_len=12)
-
-    assert chunks[0] == "alpha beta"
-    assert chunks[1].startswith("gamma")
-
-
-def test_split_message_inside_code_block_moves_before_fence():
-    content = "Intro paragraph.\n```python\nprint('a')\nprint('b')\n```\nDone"
-
-    chunks = split_message(content, max_len=35)
-
-    assert chunks[0] == "Intro paragraph.\n"
-    assert chunks[1].startswith("```python\nprint('a')")
-    assert all(chunk.count("```") % 2 == 0 for chunk in chunks[1:])
-
-
-def test_split_message_code_block_longer_than_max_len_closes_and_reopens():
-    content = "```python\n" + ("print('line one')\n" * 6) + "```\nDone"
-
-    chunks = split_message(content, max_len=60)
-
-    assert len(chunks) > 1
-    assert all(len(chunk) <= 60 for chunk in chunks)
-    assert all(chunk.count("```") % 2 == 0 for chunk in chunks)
-    assert chunks[0].startswith("```python\n")
-    assert chunks[0].endswith("\n```")
-    assert chunks[1].startswith("```python\n")
-
-
-def test_split_message_multiple_code_blocks_moves_second_block_to_next_chunk():
-    content = (
-        "First\n"
-        "```js\n"
-        "one();\n"
-        "```\n"
-        "Middle paragraph here\n"
-        "```py\n"
-        "two()\n"
-        "three()\n"
-        "```\n"
-        "End"
-    )
-
-    chunks = split_message(content, max_len=55)
-
-    assert chunks[0].endswith("Middle paragraph here\n")
-    assert chunks[1].startswith("```py\n")
-    assert all(chunk.count("```") % 2 == 0 for chunk in chunks)