feat: 实现 screen_from_messages() 从对话历史中提取儿童对话并筛查

- screener.py: 新增 screen_from_messages() 函数
  - 从消息历史提取儿童角色（role=user/child）的对话
  - 过滤元信息标记（[*] 格式）
  - 支持 base64 音频解码
  - 拼接后调用 screen() 筛查
- mcp_tool.py: 新增 psycho_screen_from_messages MCP 工具
- tests/test_screener.py: 新增 4 个测试用例覆盖新函数

tests/test_screener.py

@@ -454,3 +454,47 @@ class TestXiaozhiIntegration:
 
 
 import os  # for integration tests
+
+
+def test_screen_from_messages_empty():
+    from psycho_screener.screener import screen_from_messages
+    result = screen_from_messages([])
+    assert result["has_concern"] is False
+    assert result["severity"] == "none"
+
+
+def test_screen_from_messages_bullying():
+    from psycho_screener.screener import screen_from_messages
+    messages = [
+        {"role": "user", "content": "今天小朋友欺负我了"},
+        {"role": "assistant", "content": "发生了什么？"},
+        {"role": "user", "content": "他们打我，我好害怕"},
+    ]
+    result = screen_from_messages(messages)
+    assert result["has_concern"] is True
+    assert "bully" in result["concern_types"]
+
+
+def test_screen_from_messages_meta_filter():
+    from psycho_screener.screener import screen_from_messages
+    messages = [
+        {"role": "user", "content": "今天被打了"},
+        {"role": "assistant", "content": "嗯嗯"},
+        {"role": "user", "content": "[满意]"},
+    ]
+    result = screen_from_messages(messages)
+    # [满意] 应被过滤，只剩 "今天被打了"
+    assert result["has_concern"] is True
+
+
+def test_screen_from_messages_mixed_roles():
+    from psycho_screener.screener import screen_from_messages
+    messages = [
+        {"role": "system", "content": "你是小智"},
+        {"role": "assistant", "content": "好的主人"},
+        {"role": "user", "content": "我不想上学了"},
+        {"role": "assistant", "content": "为什么呢？"},
+        {"role": "user", "content": "同学都笑我"},
+    ]
+    result = screen_from_messages(messages)
+    assert result["has_concern"] is True