Add model methods to match backend duplicate model

The backend has an identical version of this model except for the
additional methods. This makes it so we'll be able to switch out the
backend model for this one (and therefore only update in one place)

src/cpr_sdk/pipeline_general_models.py

@@ -2,7 +2,7 @@
 from enum import Enum
 from typing import Mapping, Any, List, Optional, Sequence, Union
 
-from pydantic import BaseModel, model_validator
+from pydantic import BaseModel, field_validator, model_validator
 
 Json = dict[str, Any]
 
@@ -54,6 +54,21 @@ def convert_publication_ts_to_date(self):
 
         return self
 
+    @field_validator("type", mode="before")
+    @classmethod
+    def none_to_empty_string(cls, value):
+        """If the value is None, will convert to an empty string"""
+        return "" if value is None else value
+
+    def to_json(self) -> Mapping[str, Any]:
+        """Provide a serialisable version of the model"""
+
+        json_dict = self.model_dump()
+        json_dict["publication_ts"] = (
+            self.publication_ts.isoformat() if self.publication_ts is not None else None
+        )
+        return json_dict
+
 
 class InputData(BaseModel):
     """Expected input data containing RDS state."""

src/cpr_sdk/version.py

@@ -1,5 +1,5 @@
 _MAJOR = "1"
-_MINOR = "1"
+_MINOR = "2"
 _PATCH = "10"
 _SUFFIX = ""