fix: reject duplicate enum values and repeated non-repeatable directives

[advoretsky]

LoadSchema silently merged duplicate enum values (no uniqueness check), unlike object fields which already error with "Field X.Y can only be defined once." Add the equivalent check for enum values, reported as "Enum value X.Y can only be defined once."

Also enforce that a non-repeatable directive is applied at most once per schema location during schema validation (the executable-document rule in validator/rules already covers queries, but the type system was unchecked). The check is scoped to single authored locations via a singleLocation flag: a type's own directives are merged across its base definition and every extension, which the spec treats as distinct locations, so the merged list is exempt to avoid rejecting a directive used once per location.

Adds YAML cases covering duplicate enum values (same definition and across an extension), a repeated non-repeatable directive on a field and on an enum value, and positive cases (repeatable directive, the same directive on distinct locations, and a directive on a type plus its extension).

Fixes #435

Describe your PR and link to any relevant issues.

I have:

• [v] Added tests covering the bug / feature
• [v] Updated any relevant documentation

[coveralls]

coverage: 87.371% (+0.08%) from 87.295% — advoretsky:fix/enum-value-and-directive-uniqueness into vektah:master

[StevenACoffman]

Would you mind changing your commit to not attribute claude?

There's also a minor formatting nit:

Error: validator/schema.go:175:1: File is not properly formatted (golines)
  		if err := validateDirectives(&schema, ext.Directives, LocationSchema, nil, true); err != nil {

[advoretsky]

removed Claude attribution and fixed formatting