fix(utils): avoid hang in split_message when max_len <= 0

When max_len is 0 or negative the cut pointer never advances, so the loop hangs. Return the content unsplit, matching truncate_text_to_tokens for non-positive budgets.

nanobot/utils/helpers.py

@@ -526,6 +526,9 @@ def split_message(content: str, max_len: int = 2000) -> list[str]:
     """
     if not content:
         return []
+    # Non-positive max_len cannot advance the cut pointer; return unsplit.
+    if max_len <= 0:
+        return [content]
     if len(content) <= max_len:
         return [content]
     chunks: list[str] = []