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.

nanobot/channels/telegram.py

@@ -43,6 +43,79 @@ TELEGRAM_HTML_MAX_LEN = 4096
 TELEGRAM_REPLY_CONTEXT_MAX_LEN = TELEGRAM_MAX_MESSAGE_LEN  # Max length for reply context in user message
 
 
+def _split_telegram_markdown(content: str, max_len: int) -> list[str]:
+    """Split raw Telegram Markdown without leaving fenced code blocks unbalanced."""
+    if not content:
+        return []
+    content = content.lstrip()
+    if not content:
+        return []
+    if len(content) <= max_len:
+        return [content]
+
+    def fence_line(fence_pos: int) -> str:
+        line_end = content.find("\n", fence_pos)
+        if line_end < 0:
+            return content[fence_pos:]
+        return content[fence_pos:line_end]
+
+    def split_inside_fenced_code_block(pos: int) -> tuple[bool, int, str]:
+        if content[:pos].count("```") % 2 == 0:
+            return False, -1, ""
+        opening = content.rfind("```", 0, pos)
+        if opening < 0:
+            return True, -1, "```"
+        return True, opening, fence_line(opening)
+
+    chunks: list[str] = []
+    while content:
+        if len(content) <= max_len:
+            chunks.append(content)
+            break
+
+        cut = content[:max_len]
+        pos = cut.rfind("\n")
+        if pos <= 0:
+            pos = cut.rfind(" ")
+        if pos <= 0:
+            pos = max_len
+
+        inside_code, opening, fence = split_inside_fenced_code_block(pos)
+        if inside_code:
+            if opening > 0:
+                pos = opening
+            else:
+                closing = "\n```"
+                min_code_pos = len(fence)
+                if content.startswith(fence + "\n"):
+                    min_code_pos += 1
+                if pos < min_code_pos and min_code_pos + len(closing) > max_len:
+                    chunks.append(content[:max_len])
+                    content = content[max_len:].lstrip()
+                    continue
+                if pos + len(closing) > max_len:
+                    budget = max_len - len(closing)
+                    if budget > 0:
+                        recut = content[:budget]
+                        adjusted = recut.rfind("\n")
+                        if adjusted <= 0:
+                            adjusted = recut.rfind(" ")
+                        pos = adjusted if adjusted > 0 else budget
+                    else:
+                        closing = "```"
+                        pos = max_len - len(closing)
+                chunks.append(content[:pos] + closing)
+                remainder = content[pos:]
+                if remainder.startswith("\n"):
+                    remainder = remainder[1:]
+                content = f"{fence}\n{remainder}"
+                continue
+
+        chunks.append(content[:pos])
+        content = content[pos:].lstrip()
+    return chunks
+
+
 def _escape_telegram_html(text: str) -> str:
     """Escape text for Telegram HTML parse mode."""
     return text.replace("&", "&amp;").replace("<", "&lt;").replace(">", "&gt;")
@@ -632,7 +705,7 @@ class TelegramChannel(BaseChannel):
             # Fallback: no native keyboard → splice labels into the message so the choices survive.
             if buttons and reply_markup is None:
                 text = f"{text}\n\n{self._buttons_as_text(buttons)}"
-            chunks = split_message(text, TELEGRAM_MAX_MESSAGE_LEN)
+            chunks = _split_telegram_markdown(text, TELEGRAM_MAX_MESSAGE_LEN)
             for i, chunk in enumerate(chunks):
                 is_last = (i == len(chunks) - 1)
                 await self._send_text(
@@ -838,7 +911,7 @@ class TelegramChannel(BaseChannel):
         intermediate chunks as standalone messages, then opens a new message
         for the tail so subsequent deltas continue streaming into it.
         """
-        chunks = split_message(buf.text, TELEGRAM_MAX_MESSAGE_LEN)
+        chunks = _split_telegram_markdown(buf.text, TELEGRAM_MAX_MESSAGE_LEN)
         if len(chunks) <= 1:
             return
         try: