fix: make ontology key an optional param in cognify (#1894)

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## Description
Make ontology key optional in Swagger and None by default (it was
"string" by default before change which was causing issues when running
cognify endpoint)

## Type of Change
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- [ ] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Code refactoring
- [ ] Performance improvement
- [ ] Other (please specify):

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## Pre-submission Checklist
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- [ ] **This PR contains minimal changes necessary to address the
issue/feature**
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guidelines
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submitted already
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## Summary by CodeRabbit

* **Documentation**
* Enhanced API documentation with additional examples and validation
metadata to improve request clarity and validation guidance.

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settings.</sub>

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cognee/api/v1/cognify/routers/get_cognify_router.py

@@ -42,7 +42,9 @@ class CognifyPayloadDTO(InDTO):
         default="", description="Custom prompt for entity extraction and graph generation"
     )
     ontology_key: Optional[List[str]] = Field(
-        default=None, description="Reference to one or more previously uploaded ontologies"
+        default=None,
+        examples=[[]],
+        description="Reference to one or more previously uploaded ontologies",
     )