fix(telegram): convert markdown to HTML before splitting to avoid message length overflow

Cherry-picked from #3316 (himax12). When streaming completes in send_delta(),
the code was splitting raw markdown text by 4000, then converting to HTML.
The markdown-to-HTML conversion adds 10-33% characters, which could push
the result over Telegram's 4096 character limit.

The fix converts markdown to HTML first, then splits by 4096 (actual Telegram
limit), ensuring the edited message always fits.

Fixes #3315

nanobot/channels/telegram.py

@@ -561,14 +561,22 @@ class TelegramChannel(BaseChannel):
                     await self._remove_reaction(chat_id, int(reply_to_message_id))
                 except ValueError:
                     pass
-            chunks = split_message(buf.text, TELEGRAM_MAX_MESSAGE_LEN)
-            primary_text = chunks[0] if chunks else buf.text
+            thread_kwargs = {}
+            if message_thread_id := meta.get("message_thread_id"):
+                thread_kwargs["message_thread_id"] = message_thread_id
+            html = _markdown_to_telegram_html(buf.text)
+            if len(html) <= 4096:
+                primary_html = html
+                extra_html_chunks = []
+            else:
+                html_chunks = split_message(html, 4096)
+                primary_html = html_chunks[0]
+                extra_html_chunks = html_chunks[1:]
             try:
-                html = _markdown_to_telegram_html(primary_text)
                 await self._call_with_retry(
                     self._app.bot.edit_message_text,
                     chat_id=int_chat_id, message_id=buf.message_id,
-                    text=html, parse_mode="HTML",
+                    text=primary_html, parse_mode="HTML",
                 )
             except BadRequest as e:
                 # Only fall back to plain text on actual HTML parse/format errors.
@@ -583,7 +591,7 @@ class TelegramChannel(BaseChannel):
                     await self._call_with_retry(
                         self._app.bot.edit_message_text,
                         chat_id=int_chat_id, message_id=buf.message_id,
-                        text=primary_text,
+                        text=primary_html,
                     )
                 except Exception as e2:
                     if self._is_not_modified_error(e2):
@@ -591,10 +599,13 @@ class TelegramChannel(BaseChannel):
                     else:
                         logger.warning("Final stream edit failed: {}", e2)
                         raise  # Let ChannelManager handle retry
-            # If final content exceeds Telegram limit, keep the first chunk in
-            # the edited stream message and send the rest as follow-up messages.
-            for extra_chunk in chunks[1:]:
-                await self._send_text(int_chat_id, extra_chunk)
+            for extra_html_chunk in extra_html_chunks:
+                await self._call_with_retry(
+                    self._app.bot.send_message,
+                    chat_id=int_chat_id, text=extra_html_chunk,
+                    parse_mode="HTML",
+                    **thread_kwargs,
+                )
             self._stream_bufs.pop(chat_id, None)
             return