feat: raise at compile time when related resource's domain is missing from schema

[nallwhy]

Contributor checklist

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• I accept the AI Policy, or AI was not used in the creation of this PR.
• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

Summary

Closes #41

When a resource has a relationship to another resource whose domain is not included in the schema, AshGraphql now raises a compile-time error with a clear, actionable message — instead of silently generating a broken schema that fails at runtime with an obscure Absinthe type error.

Error message example

SourceResource has a relationship `related` to RelatedResource,
but RelatedResource's domain is not included in this schema.

Add RelatedDomain to your schema's domain list:

    use AshGraphql, domains: [SourceDomain, RelatedDomain]

Changes

• Added validate_domains_for_relationships!/2, called during use AshGraphql after all resources are collected. Checks each resource's public relationships — if the destination has the AshGraphql.Resource extension but its domain isn't in the schema, raises with instructions to fix.

[Copilot]

Pull request overview

This PR adds a compile-time validation that detects when a resource has a relationship to another resource whose domain is not included in the AshGraphql schema's domain list. Instead of silently generating a broken schema that fails at runtime with an obscure Absinthe type error, users now get a clear, actionable error message at compile time. This closes issue #41.

Changes:

• Added validate_domains_for_relationships!/2 function in AshGraphql that checks each resource's public relationships and raises if the destination resource has the AshGraphql.Resource extension but its domain is not registered in the schema.
• Integrated the validation call during use AshGraphql after all resources are collected.
• Added a test that verifies a RuntimeError is raised when a related resource's domain is missing from the schema.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
lib/ash_graphql.ex	Adds the validate_domains_for_relationships!/2 function and calls it during schema compilation to catch missing domains early.
test/missing_related_domain_test.exs	New test file verifying that a compile-time error is raised when a related resource's domain is omitted from the schema.

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