fix(graphql-relational-transformer): nullability enforcement for references relational fields

[atierian]

Description of changes

1. Validate relational fields are nullable

Relational fields MUST be nullable. These changes validate this rule.

The following is valid because the fields Primary.relatedOne, Primary.relatedMany, RelatedOne.primary and RelatedMany.primary are all nullable / non-required fields.

type Primary @model { 
  id: ID!
  relatedOne: RelatedOne @hasOne(references: "primaryId")
  relatedMany: [RelatedMany] @hasMany(references: "primaryId")
}

type RelatedOne @model { 
  id: ID!
  primaryId: ID
  primary: Primary @belongsTo(references: "primaryId")
}

type RelatedMany @model { 
  id: ID!
  primaryId: ID
  primary: Primary @belongsTo(references: "primaryId")
}

Any of these changes are invalid:

-  relatedOne: RelatedOne @hasOne(references: "primaryId")
+  relatedOne: RelatedOne! @hasOne(references: "primaryId")

-  relatedMany: [RelatedMany] @hasMany(references: "primaryId")
+  relatedMany: [RelatedMany]! @hasMany(references: "primaryId")
+  relatedMany: [RelatedMany!] @hasMany(references: "primaryId")
+  relatedMany: [RelatedMany!]! @hasMany(references: "primaryId")

-  primary: Primary @belongsTo(references: "primaryId")
+  primary: Primary! @belongsTo(references: "primaryId")

2. Validate consistent nullability of Related model's reference fields

Reference fields MUST have consistent nullability. These changes fix a bug in the existing validation implementation and improve the error message.

CDK / CloudFormation Parameters Changed

N/A

Issue #, if available

N/A

Description of how you validated changes

Unit tests

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Any CDK or CloudFormation parameter changes are called out explicitly

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[atierian]

unrelated fix in test schema