fix(agent): preserve user message in _snip_history to prevent GLM error 1214

When _snip_history truncates the message history and the only user message
ends up outside the kept window, providers like GLM reject the resulting
system→assistant sequence with error 1214 ("messages 参数非法").

Two-layer fix:
1. _snip_history now walks backwards through non_system messages to recover
   the nearest user message when none exists in the kept window.
2. _enforce_role_alternation inserts a synthetic user message
   "(conversation continued)" when the first non-system message is a bare
   assistant (no tool_calls), serving as a safety net for any edge cases
   that slip through.

Co-authored-by: darlingbud <darlingbud@users.noreply.github.com>

nanobot/agent/runner.py

@@ -932,6 +932,15 @@ class AgentRunner:
                 if message.get("role") == "user":
                     kept = kept[i:]
                     break
+            else:
+                # No user message in the kept window — walk backwards through
+                # non_system to find the nearest user message and keep it plus
+                # everything after it.  Providers like GLM reject requests
+                # where the first non-system message is not ``user`` (error 1214).
+                for idx in range(len(non_system) - 1, -1, -1):
+                    if non_system[idx].get("role") == "user":
+                        kept = non_system[idx:]
+                        break
             start = find_legal_message_start(kept)
             if start:
                 kept = kept[start:]

nanobot/providers/base.py

@@ -409,6 +409,17 @@ class LLMProvider(ABC):
             recovered["role"] = "user"
             merged.append(recovered)
 
+        # Safety net: ensure the first non-system message is not a bare
+        # ``assistant`` message.  Providers like GLM reject system→assistant
+        # with error 1214.  This can happen when upstream truncation (e.g.
+        # _snip_history) drops the only user message.  Insert a synthetic
+        # user message to keep the sequence valid.
+        for i, msg in enumerate(merged):
+            if msg.get("role") != "system":
+                if msg.get("role") == "assistant" and not msg.get("tool_calls"):
+                    merged.insert(i, {"role": "user", "content": "(conversation continued)"})
+                break
+
         return merged
 
     @staticmethod