Python: serialize the result as string, consistent with behavior before 1.2 (#7528)

### Motivation and Context

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Fix #7340

This adds a serializer to the FunctionResultContent for the result
field.

Before SK 1.2 all results where stored as string using `str(value)`,
after that was changed to Any, to allow more complex types to be stored
with full details.

This changes things back to that behavior for serialization by adding a
pydantic field_serializer for that field which uses the same method
(`str(value)`). Further this allows one to subclass
FunctionResultContent and overwrite the built-in serializer with your
own.

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

python/semantic_kernel/contents/function_result_content.py

@@ -3,7 +3,7 @@
 from typing import TYPE_CHECKING, Any, ClassVar, Literal, TypeVar
 from xml.etree.ElementTree import Element  # nosec
 
-from pydantic import Field
+from pydantic import Field, field_serializer
 from typing_extensions import deprecated
 
 from semantic_kernel.contents.const import FUNCTION_RESULT_CONTENT_TAG, TEXT_CONTENT_TAG, ContentTypes
@@ -169,3 +169,8 @@ class FunctionResultContent(KernelContent):
     def split_name(self) -> list[str]:
         """Split the name into a plugin and function name."""
         return [self.plugin_name or "", self.function_name]
+
+    @field_serializer("result")
+    def serialize_result(self, value: Any) -> str:
+        """Serialize the result."""
+        return str(value)

python/tests/unit/contents/test_chat_history.py

@@ -572,3 +572,21 @@ def test_to_from_file(chat_history: ChatHistory, tmp_path):
     assert chat_history_2.messages[2] == chat_history.messages[2]
     assert chat_history_2.messages[3] == chat_history.messages[3]
     assert chat_history_2.messages[4] == chat_history.messages[4]
+
+
+def test_chat_history_serialize(chat_history: ChatHistory):
+    class CustomResultClass:
+        def __init__(self, result):
+            self.result = result
+
+        def __str__(self) -> str:
+            return self.result
+
+    custom_result = CustomResultClass(result="CustomResultTestValue")
+    chat_history.add_system_message("You are an AI assistant")
+    chat_history.add_user_message("What is the weather in Seattle?")
+    chat_history.add_assistant_message(
+        [FunctionCallContent(id="test1", name="WeatherPlugin-GetWeather", arguments='{{ "location": "Seattle" }}')]
+    )
+    chat_history.add_tool_message([FunctionResultContent(id="test1", result=custom_result)])
+    assert "CustomResultTestValue" in chat_history.serialize()

python/tests/unit/contents/test_function_result_content.py

@@ -83,3 +83,19 @@ def test_to_cmc(unwrap: bool):
         assert cmc.items[0].text == "test-result"
     else:
         assert cmc.items[0].result == "test-result"
+
+
+def test_serialize():
+    class CustomResultClass:
+        def __init__(self, result):
+            self.result = result
+
+        def __str__(self) -> str:
+            return self.result
+
+    custom_result = CustomResultClass(result="test")
+    frc = FunctionResultContent(id="test", name="test-function", result=custom_result)
+    assert (
+        frc.model_dump_json(exclude_none=True)
+        == """{"metadata":{},"content_type":"function_result","id":"test","result":"test","name":"test-function","function_name":"function","plugin_name":"test"}"""  # noqa: E501
+    )