Add custom directive introspection and repeatable directive validation

[1e16miin]

Summary

• Fixes custom directives not appearing in GraphQL introspection queries
• Adds missing TypeSystemDirectiveLocation enum values
• Refactors directive handling for better maintainability

Changes

• Enhanced resolve-root-schema to include custom directives from schema
• Added complete __DirectiveLocation enum values (executable + type system)
• Refactored with helper functions for directive processing
• Added comprehensive tests
• Fixed existing introspection test to include new builtin directives

Test plan

• Custom directives appear in introspection results
• Built-in directives still work correctly
• DirectiveLocation enum includes all standard values
• Tests pass for directive introspection scenarios

Future improvements

The following items could be addressed in future PRs to improve directive architecture consistency:

1. Refactor directive definition structure for consistency - Currently directive definitions are scattered across parser (builtin-directives) and schema compilation (compile-directive-defs). This could be unified into a single source of truth.

2. Consolidate builtin directives across parser and schema compilation - @skip, @include are defined in parser while @deprecated is in schema compilation. All builtin directives should be managed in one place.

3. Ensure @Skip and @include are properly defined in schema directive-defs - Executable directives should also be included in schema's directive-defs for consistency, rather than being handled separately in parser.

Addresses the documentation note: "custom directives are not identified" in introspection.

[CLAassistant]

All committers have signed the CLA.

[hlship]

It's a lot to take in, but the fact that existing tests are unchanged, and the new tests identify what's been added, gives me some confidence.