Add errorMessage field to Response class

[Copilot]

The Response class had no way to communicate why a returned record is null, leaving callers without actionable context on invalid input.

Changes

CollectionGetFirstRecord_Records.cls

• Added @InvocableVariable String errorMessage to the Response inner class
• Split the combined null/empty guard into two discrete checks to surface distinct messages:
  • req.records == null → "Input list was null"
  • req.records.isEmpty() → "Input list was empty"
  • Valid input → errorMessage remains null

public class Response {
    @InvocableVariable(label='Record' description='The first record from the provided collection')
    public SObject record;
    @InvocableVariable(label='Error Message' description='Description of why no record was returned, when applicable')
    public String errorMessage;
}

CollectionGetFirstRecord_RecordsTest.cls

• Added assertions on errorMessage for empty and null input cases
• Added assertion that errorMessage is null when a valid record is returned