[BUG][typescript-fetch] TS7053 error in instanceOf methods for required parameters after 7.5.0 (reproducible in 7.9.x snapshot)

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

After version 7.5.0, instanceOf methods cause a TS7053 error for required parameters.

openapi-generator version

7.9.0-20240916.120701-53

OpenAPI declaration file content or url

Pet Store

Generation Details

Steps to reproduce

java -jar openapi-generator-cli-7.9.0-20240916.120701-53.jar generate -g typescript-fetch -i https://raw.githubusercontent.com/OpenAPITools/openapi-generator/master/modules/openapi-generator/src/test/resources/3_0/petstore.yaml -o output >> /dev/null && cat output/models/Pet.ts | grep "in value" -B 5 -A 3

/**
 * Check if a given object implements the Pet interface.
 */
export function instanceOfPet(value: object): value is Pet {
    if (!('name' in value) || value['name'] === undefined) return false;
    if (!('photoUrls' in value) || value['photoUrls'] === undefined) return false;
    return true;
}

Which causes the following TS error:

TS7053: Element implicitly has an any type because expression of type 'name' can't be used to index type {}

Related issues/PRs

It is similar to https://github.com/OpenAPITools/openapi-generator/issues/19011 but affects any required parameter.

Suggest a fix

Revert === undefined check as in version 7.5.0.

Reproduce the issue with the provided openapi-generator-cli command and the Pet Store specification, then inspect output/models/Pet.ts and compare it with output from version 7.5.0. Confirm that the generated TypeScript no longer raises TS7053 for required parameters while retaining the required-parameter checks, and verify the generated client compiles.