fix(python): [Semantic Kernel/Agent Framework] 保留 None 默认值

- 依据参数必填元数据构造 Pydantic 字段
- 覆盖显式 None 与非 None 两类可选默认值

Author: ump45nose

python/semantic_kernel/functions/kernel_function.py

@@ -447,13 +447,15 @@ def as_agent_framework_tool(
         fields = {}
         for param in self.parameters:
             if param.include_in_function_choices:
-                if param.default_value is not None:
+                # Requiredness is explicit metadata: None can be a legitimate
+                # default for optional parameters and must remain a Pydantic default.
+                if param.is_required:
+                    fields[param.name] = (param.type_, Field(description=param.description))
+                else:
                     fields[param.name] = (
                         param.type_,
                         Field(description=param.description, default=param.default_value),
                     )
-                else:
-                    fields[param.name] = (param.type_, Field(description=param.description))
         input_model = create_model("InputModel", **fields)  # type: ignore
 
         async def wrapper(*args: Any, **kwargs: Any) -> Any:

python/tests/unit/functions/test_kernel_function_from_method.py

@@ -148,15 +148,18 @@ def __init__(self, *, input_model: Any, **kwargs: Any):
     monkeypatch.setitem(__import__("sys").modules, "agent_framework", Mock(AIFunction=MockAIFunction))
 
     @kernel_function()
-    def function(count: int = 5) -> int:
+    def function(count: int = 5, label: str | None = None) -> int:
         return count
 
     native_function = KernelFunction.from_method(method=function, plugin_name="MockPlugin")
     tool = native_function.as_agent_framework_tool(kernel=Mock())
-    field = tool.input_model.model_fields["count"]
+    count_field = tool.input_model.model_fields["count"]
+    label_field = tool.input_model.model_fields["label"]
 
-    assert field.default == 5
-    assert not field.is_required()
+    assert count_field.default == 5
+    assert not count_field.is_required()
+    assert label_field.default is None
+    assert not label_field.is_required()
 
 
 def test_init_method_is_none():