Add | null to oneOf union type when Null variant is filtered

When a oneOf includes type: 'null', the Null model reference is
removed to prevent broken imports. The union type should include
| null to preserve the nullable semantics from the original spec.

Author: guizmaii

modules/openapi-generator/src/main/resources/typescript-fetch/modelOneOfInterfaces.mustache

@@ -3,4 +3,4 @@
  * {{#lambda.indented_star_1}}{{{unescapedDescription}}}{{/lambda.indented_star_1}}
  * @export
  */
-export type {{classname}} = {{#discriminator}}{{#mappedModels}}{ {{discriminator.propertyName}}: '{{mappingName}}' } & {{modelName}}{{^-last}} | {{/-last}}{{/mappedModels}}{{/discriminator}}{{^discriminator}}{{#oneOf}}{{{.}}}{{^-last}} | {{/-last}}{{/oneOf}}{{/discriminator}};
+export type {{classname}} = {{#discriminator}}{{#mappedModels}}{ {{discriminator.propertyName}}: '{{mappingName}}' } & {{modelName}}{{^-last}} | {{/-last}}{{/mappedModels}}{{/discriminator}}{{^discriminator}}{{#oneOf}}{{{.}}}{{^-last}} | {{/-last}}{{/oneOf}}{{/discriminator}}{{#isNullable}} | null{{/isNullable}};

modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java

@@ -462,8 +462,8 @@ public void testNullableOneOfDoesNotImportNullModel() throws IOException {
         // Should contain the valid model types
         TestUtils.assertFileContains(nullableResult, "FileLocation");
         TestUtils.assertFileContains(nullableResult, "DetailedLocation");
-        // Union type should not include Null
-        TestUtils.assertFileContains(nullableResult, "export type NullableResult = DetailedLocation | FileLocation");
+        // Union type should not include Null but should be nullable
+        TestUtils.assertFileContains(nullableResult, "export type NullableResult = DetailedLocation | FileLocation | null");
 
         // No Null.ts model file should be generated
         TestUtils.assertFileNotExists(Paths.get(output + "/models/Null.ts"));