fix: support backwards compatible changes in discriminators

Closes #20982

Author: aeneasr

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

@@ -126,7 +126,7 @@ export function {{classname}}ToJSONTyped(value?: {{#hasReadOnly}}Omit<{{classnam
                 return {{modelName}}ToJSONTyped(value as {{modelName}}, ignoreDiscriminator);
         {{/discriminator.mappedModels}}
             default:
-                throw new Error(`No variant of {{classname}} exists with '{{discriminator.propertyName}}=${value['{{discriminator.propertyName}}']}'`);
+                return value;
         }
     }
     {{/discriminator}}

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

@@ -27,7 +27,7 @@ export function {{classname}}FromJSONTyped(json: any, ignoreDiscriminator: boole
             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}}=${json['{{discriminator.propertyName}}']}'`);
+            return json;
     }
 {{/discriminator}}
 {{^discriminator}}
@@ -56,7 +56,7 @@ export function {{classname}}ToJSONTyped(value?: {{classname}} | null, ignoreDis
             return Object.assign({}, {{modelName}}ToJSON(value), { {{discriminator.propertyName}}: '{{mappingName}}' } as const);
 {{/discriminator.mappedModels}}
         default:
-            throw new Error(`No variant of {{classname}} exists with '{{discriminator.propertyName}}=${value['{{discriminator.propertyName}}']}'`);
+            return json;
     }
 {{/discriminator}}