feat(gqlerrors): support multiple errors from resolver via errors.Join

[haru0017]

Issue

N/A

TL; DR

• Enable resolvers to return multiple GraphQL errors using errors.Join

Task summary / Change details

• Add FormatErrorsFromError function in gqlerrors/formatted.go to expand joined errors
• Update handleFieldError in executor.go to use the new function
• Add tests for multiple errors and nested joined errors

Usage:

func resolve(p graphql.ResolveParams) (interface{}, error) {
    err1 := errors.New("first error")
    err2 := errors.New("second error")
    return nil, errors.Join(err1, err2)
}

Result:

{
  "errors": [
    {"message": "first error", "path": ["field"]},
    {"message": "second error", "path": ["field"]}
  ]
}

Backward compatible - single errors work as before.

[k1LoW]

GREAT! LTGM!