feat: enforce conditional payment validation in schema

schema.py

@@ -1,6 +1,6 @@
 from typing import Literal, Optional
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, model_validator
 
 
 class DocumentInput(BaseModel):
@@ -13,4 +13,15 @@ class DocumentInput(BaseModel):
     fixed_fee_gbp: Optional[float] = Field(None, gt=0)
 
     include_confidentiality: bool = False
-    governing_law: str
+    governing_law: str
+
+    @model_validator(mode="after")
+    def check_payment_fields(self):
+        # enforce conditional reqs cleanly
+        if self.payment_type == "hourly" and self.hourly_rate_gbp is None:
+            raise ValueError("hourly_rate_gbp is required when payment_type is 'hourly'")
+
+        if self.payment_type == "fixed" and self.fixed_fee_gbp is None:
+            raise ValueError("fixed_fee_gbp is required when payment_type is 'fixed'")
+
+        return self