Support custom data type for DataTypeAttribute

[tayfunyuksel]

Adds support for the DataTypeAttribute when a custom data type string is provided (e.g., [DataType("uuid")]). This will set the format property on the corresponding OpenAPI schema.

A new unit test is included to verify this behavior.

Pull Request

Adds support for mapping custom DataType attributes to the format property of an OpenAPI schema.

The issue or feature being addressed

Currently, Swashbuckle.AspNetCore maps well-known System.ComponentModel.DataAnnotations.DataType enum values (like DateTime, Password, EmailAddress) to their respective OpenAPI formats. However, it did not support custom string-based DataType values.

This PR checks if DataTypeAttribute.DataType is AnnotationsDataType.Custom. If it is, it assigns the value of DataTypeAttribute.CustomDataType to the schema.Format.

According to the OpenAPI 3.0.0 Specification - Data Types, the format property is an open string-valued property that can have any value. The spec explicitly notes:

OpenAPI Specification - Data Types
"Primitives have an optional modifier property: format. OAS uses several known formats to define in fine detail the data type being used. However, to support documentation needs, the format property is an open string-valued property, and can have any value. Formats such as "email", "uuid", and so on, MAY be used even though undefined by this specification. Types that are not accompanied by a format property follow the type definition in the JSON Schema. Tools that do not recognize a specific format MAY default back to the type alone, as if the format is not specified."

Details on the issue fix or feature implementation

With this change, developers can easily add custom formats to their properties like so:

[DataType("uuid")]
public string Id { get; set; }

Which will correctly generate the following OpenAPI schema:

{
  "type": "string",
  "format": "uuid"
}

[martincostello]

Could you add-to/update the integration tests so we have at least one snapshot that verifies that the custom format is serialized to the OpenAPI document?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.01%. Comparing base (265e280) to head (722bf25).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

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  Coverage   95.01%   95.01%           
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  Files         111      111           
  Lines        3910     3912    +2     
  Branches      788      789    +1     
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+ Hits         3715     3717    +2     
  Misses        195      195           

Flag	Coverage Δ
Linux	95.01% <100.00%> (+<0.01%)	⬆️
Windows	95.01% <100.00%> (+<0.01%)	⬆️
macOS	95.01% <100.00%> (+<0.01%)	⬆️

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