feat(gqlerrors): support custom path per error via PathError interface

[haru0017]

Issue

N/A

TL; DR

• Enable errors to specify custom paths, overriding the default field path

Task summary / Change details

• Add PathError interface in gqlerrors/formatted.go
• Update FormatError to check for PathError and use custom path
• Add tests for custom paths, combined with extensions, and multiple errors

Usage:

type ValidationError struct {
    Msg       string
    FieldPath []interface{}
}

func (e *ValidationError) Error() string       { return e.Msg }
func (e *ValidationError) Path() []interface{} { return e.FieldPath }

func resolve(p graphql.ResolveParams) (interface{}, error) {
    return nil, &ValidationError{
        Msg:       "Name is required",
        FieldPath: []interface{}{"createUser", "input", "name"},
    }
}

Result:

{
  "errors": [
    {"message": "Name is required", "path": ["createUser", "input", "name"]}
  ]
}

Backward compatible - errors without PathError work as before.

[k1LoW]

LGTM!