feat: parse EXTRA_BODY as a JSON object per source

tests/unit/test_config.py

@@ -103,6 +103,35 @@ class TestSourceAggregation:
             GatewaySettings.from_env("LLM", env=env)
 
 
+class TestExtraBodyParsing:
+    """配置级采样参数的 env 解析(issue #4 Task 2)。"""
+
+    def test_json_object_parsed(self):
+        env = _env(**{"LLM__QWEN__1__EXTRA_BODY": '{"temperature": 0, "seed": 42}'})
+        s = GatewaySettings.from_env("LLM", env=env)
+        assert s.sources[0].extra_body == {"temperature": 0, "seed": 42}
+
+    def test_absent_defaults_to_empty(self):
+        assert GatewaySettings.from_env("LLM", env=_env()).sources[0].extra_body == {}
+
+    def test_invalid_json_fails_loudly(self):
+        env = _env(**{"LLM__QWEN__1__EXTRA_BODY": "{invalid"})
+        with pytest.raises(ValueError, match="EXTRA_BODY"):
+            GatewaySettings.from_env("LLM", env=env)
+
+    def test_non_object_json_fails(self):
+        """数组/标量都不是请求体片段,静默接受会让参数悄悄不生效。"""
+        env = _env(**{"LLM__QWEN__1__EXTRA_BODY": "[1, 2]"})
+        with pytest.raises(ValueError, match="JSON 对象"):
+            GatewaySettings.from_env("LLM", env=env)
+
+    def test_protected_key_rejected_through_assembly(self):
+        """校验确实挂在装配路径上(而非只在 types.py 里孤立存在)。"""
+        env = _env(**{"LLM__QWEN__1__EXTRA_BODY": '{"model": "sneaky"}'})
+        with pytest.raises(ValueError, match="model"):
+            GatewaySettings.from_env("LLM", env=env)
+
+
 class TestResilienceKeys:
     def test_flat_legacy_keys(self):
         s = GatewaySettings.from_env("LLM", env=_env())