fix(agent): reject argless action in normalization; add boundary test

核心算法 #10（Agent Loop）：修复 Codex 质量审查 Critical——
_normalize_action 仅在除 tool 外至少存在一个平铺参数键时才收拢，
{"tool": "x"} 无参结构不再被静默升级为空 args 合法结构，照旧
返回 None 走 retry 追问路径。补边界测试 + 测试辅助方法类型注解
与中文 docstring。

tests/unit/test_agent_loop.py

@@ -263,7 +263,8 @@ _REAL_FLAT_FENCED = """{
 class TestParseNormalization:
     """deepseek 输出变体（args 平铺 + ```json 围栏）归一化。"""
 
-    def _parse(self, content: str):
+    def _parse(self, content: str) -> tuple[str, dict, dict, str, dict, str] | None:
+        """构造 AgentLoop 并解析给定 content，返回 _parse_response 结果。"""
         loop = AgentLoop(llm=AsyncMock(), max_steps=10)
         return loop._parse_response(_make_response(content))
 
@@ -279,6 +280,7 @@ class TestParseNormalization:
         }
 
     def test_leading_json_fence(self) -> None:
+        """开头 ```json 围栏 + 尾部围栏包裹的标准结构可正常解析。"""
         content = '```json\n{"reflect": {}, "plan": {}, "action": {"tool": "submit_answer", "args": {"answer": "A"}}}\n```'
         parsed = self._parse(content)
         assert parsed is not None
@@ -291,8 +293,14 @@ class TestParseNormalization:
         assert parsed[4] == {"tool": "submit_answer", "args": {"answer": "B"}}
 
     def test_action_missing_tool_still_rejected(self) -> None:
+        """action 缺 tool 键的结构仍被拒绝。"""
         content = json.dumps({"reflect": {}, "plan": {}, "action": {"node_id": "x"}})
         assert self._parse(content) is None
 
+    def test_argless_action_still_rejected(self) -> None:
+        """有 tool、无 args、无平铺参数键 → 不得收拢为空 args，必须拒绝。"""
+        content = json.dumps({"reflect": {}, "plan": {}, "action": {"tool": "submit_answer"}})
+        assert self._parse(content) is None
+
     def test_empty_content_still_rejected(self) -> None:
         assert self._parse("") is None