Make Relationship.type optional

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jochenchrist

src/open_data_contract_standard/model.py

@@ -116,7 +116,7 @@ class Description(pyd.BaseModel):
 
 
 class Relationship(pyd.BaseModel):
-    type: str | None
+    type: str | None = None
     from_: str | list[str] | None = pyd.Field(default=None, alias="from")
     to: str | list[str] | None = None
     customProperties: list[CustomProperty] | None = None