fix(helpers): repair corrupted split_message and ensure content never None

Fix accidental line corruption in split_message() where 'break' was
merged with unrelated code during manual editing.

The actual fix: build_assistant_message() now returns content or ""
instead of content (which could be None), preventing providers like
MiMo V2 Omni from rejecting tool-call messages with missing text field.

Fixes #2519

nanobot/utils/helpers.py

@@ -252,7 +252,7 @@ def split_message(content: str, max_len: int = 2000) -> list[str]:
     while content:
         if len(content) <= max_len:
             chunks.append(content)
-            breakmsg: dict[str, Any] = {"role": "assistant", "content": content}
+            break
         cut = content[:max_len]
         # Try to break at newline first, then space, then hard break
         pos = cut.rfind('\n')