fix: strip hallucinated <tool_result> blocks in ReACT loop

Prevents LLM-fabricated tool results from being appended to conversation
history. Adds _sanitize_tool_results() and applies it to both
_generate_section_react() and chat() code paths.

System prompt is hardened with an explicit warning against fabricating
<tool_result> content.

Closes #529

backend/app/services/report_agent.py

@@ -732,6 +732,7 @@ SECTION_SYSTEM_PROMPT_TEMPLATE = """\
 
 ⚠️ 严格禁止：
 - 禁止在一次回复中同时包含工具调用和 Final Answer
+- 禁止自行编写 <tool_result> 内容。工具结果会由系统真实执行后返回。如果你在回复中包含 <tool_result> 块（无论是否包含内容），系统将自动将其从对话历史中删除，不会被视为真实工具执行结果。
 - 禁止自己编造工具返回结果（Observation），所有工具结果由系统注入
 - 每次回复最多调用一个工具
 
@@ -1218,6 +1219,14 @@ class ReportAgent:
                 ]
             )
     
+    @staticmethod
+    def _sanitize_tool_results(response: str) -> str:
+        """移除 LLM 自行编造的 <tool_result> 块，防止虚假内容污染消息历史。"""
+        cleaned = re.sub(r'<tool_result>.*?</tool_result>', '', response, flags=re.DOTALL)
+        if cleaned != response:
+            logger.warning("Stripped hallucinated <tool_result> block(s) from LLM response")
+        return cleaned
+
     def _generate_section_react(
         self, 
         section: ReportSection,
@@ -1308,6 +1317,10 @@ class ReportAgent:
                 max_tokens=4096
             )
 
+            # 清洗：移除 LLM 自行编造的 <tool_result> 块（防止虚假内容污染消息历史）
+            if response:
+                response = self._sanitize_tool_results(response)
+
             # 检查 LLM 返回是否为 None（API 异常或内容为空）
             if response is None:
                 logger.warning(t('report.sectionIterNone', title=section.title, iteration=iteration + 1))
@@ -1829,9 +1842,12 @@ class ReportAgent:
                 messages=messages,
                 temperature=0.5
             )
+            if response:
+                response = self._sanitize_tool_results(response)
 
             # 解析工具调用
             tool_calls = self._parse_tool_calls(response)
+            tool_calls = self._parse_tool_calls(response)
             
             if not tool_calls:
                 # 没有工具调用，直接返回响应
@@ -1869,6 +1885,8 @@ class ReportAgent:
             messages=messages,
             temperature=0.5
         )
+        if final_response:
+            final_response = self._sanitize_tool_results(final_response)
 
         # 清理响应
         clean_response = re.sub(r'<tool_call>.*?</tool_call>', '', final_response, flags=re.DOTALL)