Update OpenAPIKit to v4

Package.swift

@@ -45,8 +45,8 @@ let package = Package(
         .package(url: "https://github.com/apple/swift-collections", from: "1.1.4"),
 
         // Read OpenAPI documents
-        .package(url: "https://github.com/mattpolzin/OpenAPIKit", from: "3.9.0"),
-        .package(url: "https://github.com/jpsim/Yams", "4.0.0"..<"7.0.0"),
+        .package(url: "https://github.com/mattpolzin/OpenAPIKit", from: "4.3.1"),
+        .package(url: "https://github.com/jpsim/Yams", "5.1.0"..<"7.0.0"),
 
         // CLI Tool
         .package(url: "https://github.com/apple/swift-argument-parser", from: "1.3.0"),

Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateRawEnum.swift

@@ -100,7 +100,9 @@ extension FileTranslator {
                 // In nullable enum schemas, empty strings are parsed as Void.
                 // This is unlikely to be fixed, so handling that case here.
                 // https://github.com/apple/swift-openapi-generator/issues/118
-                if isNullable && anyValue is Void {
+                // Also handle nil values in nullable schemas.
+                let isNullValue = anyValue is Void || (anyValue as? String) == nil
+                if isNullable && isNullValue {
                     try addIfUnique(id: .string(""), caseName: context.safeNameGenerator.swiftMemberName(for: ""))
                 } else {
                     guard let rawValue = anyValue as? String else {

Tests/OpenAPIGeneratorCoreTests/Parser/Test_YamsParser.swift

@@ -126,7 +126,7 @@ final class Test_YamsParser: Test_Core {
             /foo.yaml: error: Found neither a $ref nor a PathItem in Document.paths['/system']. 
 
             PathItem could not be decoded because:
-            Inconsistency encountered when parsing `Vendor Extension` for the **GET** endpoint under `/system`: Found at least one vendor extension property that does not begin with the required 'x-' prefix. Invalid properties: [ resonance ]..
+            Problem encountered when parsing `Vendor Extension` for the **GET** endpoint under `/system`: Found at least one vendor extension property that does not begin with the required 'x-' prefix. Invalid properties: [ resonance ]..
             """
         assertThrownError(try _test(yaml), expectedDiagnostic: expected)
     }

Tests/OpenAPIGeneratorCoreTests/Translator/TypeAssignment/Test_TypeMatcher.swift

@@ -245,7 +245,7 @@ final class Test_TypeMatcher: Test_Core {
     static let multipartElementTypeReferenceIfReferenceableTypes:
         [(UnresolvedSchema?, OrderedDictionary<String, OpenAPI.Content.Encoding>?, String?)] = [
             (nil, nil, nil), (.b(.string), nil, nil), (.a(.component(named: "Foo")), nil, "Foo"),
-            (.a(.component(named: "Foo")), ["foo": .init(contentType: .json)], nil),
+            (.a(.component(named: "Foo")), ["foo": .init(contentTypes: [.json])], nil),
         ]
     func testMultipartElementTypeReferenceIfReferenceableTypes() throws {
         for (schema, encoding, name) in Self.multipartElementTypeReferenceIfReferenceableTypes {

Tests/OpenAPIGeneratorCoreTests/Translator/TypesTranslator/Test_translateSchemas.swift

@@ -35,7 +35,7 @@ class Test_translateSchemas: Test_Core {
             (
                 schemaWithWarnings,
                 [
-                    "warning: Schema warning: Inconsistency encountered when parsing `OpenAPI Schema`: Found schema attributes not consistent with the type specified: string. Specifically, attributes for these other types: [\"array\"]. [context: codingPath=, contextString=, subjectName=OpenAPI Schema]"
+                    "warning: Schema warning: Problem encountered when parsing `OpenAPI Schema`: Found schema attributes not consistent with the type specified: string. Specifically, attributes for these other types: [\"array\"]. [context: codingPath=, contextString=, subjectName=OpenAPI Schema]"
                 ]
             ),
         ]