fix(openapi): convert type null and nullable array types to OpenAPI 3.0 nullable schemas

[lx3133584]

Problem

When using JSON Schemas containing type: "null" (e.g. generated by TypeBox Type.Null() or nullable type unions), OpenAPI 3.0 validation fails with type must be equal to one of the allowed values because "null" is not a primitive type keyword in OpenAPI 3.0.

Root Cause

convertJsonSchemaToOpenapi3 did not transform JSON Schema type: "null" or array types containing "null" into OpenAPI 3.0 { nullable: true } syntax.

Fix

• In convertJsonSchemaToOpenapi3, converted type: "null" to { nullable: true }.
• Converted array types containing "null" (e.g. type: ["string", "null"]) into single type / anyOf with nullable: true.

Testing

• Added unit tests in test/spec/openapi/schema.test.js validating schema conversion with Swagger.validate().
• Verified 100% test coverage and TypeScript type tests pass.

[Copilot]

Pull request overview

Adds JSON Schema null-type conversion for OpenAPI 3.0.

Changes:

• Converts null and nullable union types.
• Adds validation and output assertions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/spec/openapi/utils.js	Implements null-type conversion.
test/spec/openapi/schema.test.js	Tests converted response schemas.

Suppressed comments (1)

lib/spec/openapi/utils.js:567

• In OpenAPI 3.0, nullable only adds null when type is explicitly defined in the same Schema Object. These branches delete type, so the multi-type result still rejects null, while the null-only result becomes unconstrained and accepts every value. Put nullable on a typed anyOf branch for multi-type unions and use a null-restricting representation (for example an enum/typed nullable branch) for null-only schemas; the test should validate example values because Swagger.validate() checks document structure, not instance semantics.

        } else if (remainingTypes.length > 1) {
          openapiSchema.anyOf = remainingTypes.map(t => ({ type: t }))
          delete openapiSchema.type
        } else {
          delete openapiSchema.type

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