[typescript-fetch] fix #18876 Type error in oneOf when legacyDiscriminatorBehavior is false (#18877)

Author: fa0311

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

@@ -24,7 +24,7 @@ export function {{classname}}FromJSONTyped(json: any, ignoreDiscriminator: boole
     switch (json['{{discriminator.propertyBaseName}}']) {
 {{#discriminator.mappedModels}}
         case '{{mappingName}}':
-            return Object.assign({}, {{modelName}}FromJSONTyped(json, true), { {{discriminator.propertyName}}: '{{mappingName}}' });
+            return Object.assign({}, {{modelName}}FromJSONTyped(json, true), { {{discriminator.propertyName}}: '{{mappingName}}' } as const);
 {{/discriminator.mappedModels}}
         default:
             throw new Error(`No variant of {{classname}} exists with '{{discriminator.propertyName}}=${json['{{discriminator.propertyName}}']}'`);