Add NonNull attribute to fields which cannot be null when queried from the API

[matnun-br]

No description provided.

[Copilot]

Pull Request Overview

This PR introduces a NonNull attribute to explicitly mark GraphQL fields that cannot be null when queried, improving schema expressiveness. Key changes include:

• Appending the [NonNull] attribute during type generation if the field type is NON_NULL.
• Adding a NonNullAttribute implementation in the Abstractions layer.
• Updating various GraphQL API model classes (including tests) to annotate appropriate properties with [NonNull].

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
GraphQLSharp/TypeGenerator/GraphQLTypeGenerator.cs	Appends the [NonNull] attribute for NON_NULL fields during generation
GraphQLSharp/Abstractions/NonNullAttribute.cs	Defines the NonNull attribute
GraphQLSharp.Tests/square.cs	Updates GraphQL model classes to include the [NonNull] attribute

Comments suppressed due to low confidence (2)

GraphQLSharp/TypeGenerator/GraphQLTypeGenerator.cs:301

• Consider adding a clarifying comment here that explains the purpose of appending the [NonNull] attribute, and how this annotation aligns with the C# nullable reference types for future maintainability.

        if (f.type.kind == GraphQLTypeKind.NON_NULL)

GraphQLSharp.Tests/square.cs:43

• When marking a property as NonNull, consider using a non-nullable reference type (i.e. removing the '?' from the type) if the field truly can never be null, to better reflect the intended contract in C#.

        [NonNull]