fix(agent): normalize fenced and flat-args LLM outputs in parser

核心算法 #10（Agent Loop）：仅加固 _parse_response 解析路径——
剥除 ```json 围栏 + 收拢 action 平铺参数（deepseek 稳定输出变体，
案例 637-3/615-3 三连拒 0 步阵亡）。Thinking+JSON 协议、json_repair
兜底链、hook 时序与步数语义均未改动。

tests/unit/test_agent_loop.py

@@ -239,3 +239,60 @@ class TestAgentLoop:
         assert "after_tool:0" in tracker.events
         assert "after_step:0" in tracker.events
         assert "on_finish:finished" in tracker.events
+
+
+# ── A1 解析容错测试（Spec-1）──────────────────────────────────
+
+# 生产真实样本结构：尾部围栏残留 + action.args 平铺（开头围栏场景由
+# test_leading_json_fence 单独覆盖）
+_REAL_FLAT_FENCED = """{
+  "plan": {
+    "goal": "从三个L1根节点开始建立全局认知",
+    "tool": "view_node",
+    "reason": "三个L1节点覆盖整个视频"
+  },
+  "action": {
+    "tool": "view_node",
+    "node_id": "J5Npf2xJpag_L1_000",
+    "question": "What is the overall topic of this video?"
+  }
+}
+```"""
+
+
+class TestParseNormalization:
+    """deepseek 输出变体（args 平铺 + ```json 围栏）归一化。"""
+
+    def _parse(self, content: str):
+        loop = AgentLoop(llm=AsyncMock(), max_steps=10)
+        return loop._parse_response(_make_response(content))
+
+    def test_flat_args_with_trailing_fence(self) -> None:
+        """生产样本：action 平铺 node_id/question + 尾部围栏。"""
+        parsed = self._parse(_REAL_FLAT_FENCED)
+        assert parsed is not None
+        action = parsed[4]
+        assert action["tool"] == "view_node"
+        assert action["args"] == {
+            "node_id": "J5Npf2xJpag_L1_000",
+            "question": "What is the overall topic of this video?",
+        }
+
+    def test_leading_json_fence(self) -> None:
+        content = '```json\n{"reflect": {}, "plan": {}, "action": {"tool": "submit_answer", "args": {"answer": "A"}}}\n```'
+        parsed = self._parse(content)
+        assert parsed is not None
+        assert parsed[4]["args"] == {"answer": "A"}
+
+    def test_nested_args_unchanged(self) -> None:
+        """标准嵌套结构不受归一化影响。"""
+        parsed = self._parse(_submit_json("B"))
+        assert parsed is not None
+        assert parsed[4] == {"tool": "submit_answer", "args": {"answer": "B"}}
+
+    def test_action_missing_tool_still_rejected(self) -> None:
+        content = json.dumps({"reflect": {}, "plan": {}, "action": {"node_id": "x"}})
+        assert self._parse(content) is None
+
+    def test_empty_content_still_rejected(self) -> None:
+        assert self._parse("") is None