feat: Add flow_id serialization in Message class

This commit adds the `serialize_flow_id` method to the `Message` class in the `message.py` file. This method serializes the `flow_id` attribute of a `Message` object, converting it to a UUID if it is a string. This ensures consistent serialization of the `flow_id` attribute when working with the `Message` class.

src/backend/base/langflow/schema/message.py

@@ -41,6 +41,12 @@ class Message(Data):
             value = str(value)
         return value
 
+    @field_serializer("flow_id")
+    def serialize_flow_id(value):
+        if isinstance(value, str):
+            return UUID(value)
+        return value
+
     @field_validator("files", mode="before")
     @classmethod
     def validate_files(cls, value):