feat(gen2-migration): add migration support for overriding default reso…

[dgandhi62]

When a Gen1 project has custom VTL resolver files in amplify/backend/api/<name>/resolvers/, the migration tool now detects them, copies them to amplify/data/resolvers/ in the Gen2 output, and generates backend.ts code that overrides the pipeline function mapping templates at runtime.

Gen1 and Gen2 both use pipeline resolvers. The generated code reads VTL files from data/resolvers/, looks up the corresponding cfnFunctionConfigurations entry by convention (${typeName}${FieldName}DataResolverFn), clears the S3 template location, and sets the inline template content instead. Both request (.req.vtl) and response (.res.vtl) overrides are supported.

EXPECTED OUTPUT

import * as assets from 'aws-cdk-lib/aws-s3-assets'
const __dirname = dirname(fileURLToPath(import.meta.url));
const resolversDir = join(__dirname, 'data/resolvers');
const resolverFiles = readdirSync(resolversDir).filter(
  (f) => f.endsWith('.req.vtl') || f.endsWith('.res.vtl')
);
for (const file of resolverFiles) {
  const parts = file.replace('.req.vtl', '').replace('.res.vtl', '').split('.');
  const [typeName, fieldName] = parts;
  const isRequest = file.endsWith('.req.vtl');
  const functionId = `${typeName}${
    fieldName.charAt(0).toUpperCase() + fieldName.slice(1)
  }DataResolverFn`;
  const pipelineFunction =
    backend.data.resources.cfnResources.cfnFunctionConfigurations[functionId];
  if (pipelineFunction) {
    const templatePath = join(resolversDir, file);
    const vtlTemplate = new assets.Asset(backend.data.stack, `VTLTemplate-${file}`, {
      path: templatePath
    });
    if (isRequest) {
      pipelineFunction.requestMappingTemplateS3Location = vtlTemplate.s3ObjectUrl;
    } else {
      pipelineFunction.responseMappingTemplateS3Location = vtlTemplate.s3ObjectUrl;
    }
  }
}

Along with a copied resolvers directory