feat: add validateFullSchema and assertValidFullSchema

[vishwakt]

Motivation

Closes #4415.

validateSchema() validates a source schema and rejects any name beginning with __, since those are reserved by GraphQL introspection. However, tools that work with a full schema (for example one reconstructed from an introspection result, as in GraphiQL or codegen) may legitimately encounter reserved names, including ones added by a newer version of GraphQL than the running version of graphql-js implements.

The reproducer from the issue fails today purely on the reserved-name rule, even though the SDL is otherwise valid:

const schema = buildSchema(`
  type Query { hello: String }

  enum __ErrorBehavior { NULL PROPAGATE HALT }

  directive @behavior(onError: __ErrorBehavior) on SCHEMA
`);

validateSchema(schema);
// GraphQLError: Name "__ErrorBehavior" must not begin with "__", which is reserved by GraphQL introspection.

See the Full Schemas RFC for the broader context on source vs. full schemas.

Changes

• Add experimentalValidateFullSchema(schema) and experimentalAssertValidFullSchema(schema), mirroring validateSchema / assertValidSchema.
• They run every existing type-system check except the reserved-name (__) restriction, so a full schema validates without spurious errors.
• Internally, both modes share the existing validation pipeline via a single allowReservedNames flag threaded through SchemaValidationContext.
• The two modes use independent validation caches (__validationErrors vs. __fullSchemaValidationErrors) so their results never collide regardless of call order.
• The functions use the experimental name prefix (matching experimentalExecuteIncrementally) to signal the API may change while behavior and test coverage for full schemas converge.

Tests

Added coverage for: the issue's reproducer, reserved names on types/fields/args/enum values/input fields, that non-name errors are still reported when validating a full schema, independent caching, and the assert variant.

npm run check, npm run lint, and the full test suite pass locally.

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[martinbonnin]

LGTM although not a graphql-js expert.

Might be worth marking this function as experimental until we collectivelly converge on a bigger test suite?

[vishwakt]

Good idea, thanks. I marked both functions as experimental by adopting the existing experimental name prefix convention (as used by experimentalExecuteIncrementally / experimentalExecuteRootSelectionSet), since that seems to be the established pattern in the repo for signalling an unstable API rather than a JSDoc tag. They're now experimentalValidateFullSchema and experimentalAssertValidFullSchema, and I added a note in the JSDoc too. Happy to revisit the names once the behavior and test coverage for full schemas converge.

[martinbonnin]

LGTM. Thanks for addressing the feedback.

Pinging @yaacovCR @benjie @jerelmiller for more expert eyes.