fix(@nestjs/graphql): keep class directive when a field has the same SDL

[yogeshwaran-c]

Summary

• Fix addDirectiveMetadata silently dropping a class-level @Directive(sdl) whenever any field on the same class already has a property @Directive(sdl) with the same SDL.
• Dedupe against the class's own directive list instead, preserving the original "no exact duplicates" intent but without the cross-contamination between the class and its fields.

Bug

@ObjectType()
@Directive('@tag(name: "public")')
class Account {
  @Field(() => ID) id: string;

  @Field()
  @Directive('@tag(name: "public")')
  email: string;
}

Because decorators run field-before-class, fieldDirectives.sdls already contained @tag(name: "public") by the time the class decorator fired, and the class directive was skipped. The emitted SDL therefore lacked the class-level @tag, which breaks Apollo Contracts / federation tagging patterns that tag both the entity and one of its fields.

Fix

Check classMetadata.classDirectives for an existing SDL match instead of the field-directive set. This still prevents stacking the exact same class directive twice on the same target, but decouples the check from field-directive state.

Test plan

• New spec class-directive-not-shadowed.spec.ts fails on master; passes after the fix.
• tests/schema-builder/** continues to pass (including the directive inheritance coverage from fix(@nestjs/graphql): inherit class directives from abstract parents #3938).