fix: prevent session poisoning from null/error LLM responses

When an LLM returns content: null on a plain assistant message (no
tool_calls), the null gets saved to session history and causes
permanent 400 errors on every subsequent request.

- Sanitize None content on plain assistant messages to "(empty)" in
  _sanitize_empty_content(), matching the existing empty-string handling
- Skip persisting error responses (finish_reason="error") to the
  message history in _run_agent_loop(), preventing poison loops

Closes #1303

nanobot/agent/loop.py

@@ -224,6 +224,12 @@ class AgentLoop:
                     )
             else:
                 clean = self._strip_think(response.content)
+                # Don't persist error responses to session history — they can
+                # poison the context and cause permanent 400 loops (#1303).
+                if response.finish_reason == "error":
+                    logger.error("LLM returned error: {}", (clean or "")[:200])
+                    final_content = clean or "Sorry, I encountered an error calling the AI model."
+                    break
                 messages = self.context.add_assistant_message(
                     messages, clean, reasoning_content=response.reasoning_content,
                 )