Fix @key directive missing on deferred indexed types

[jwils]

Summary

• Types created in after_user_definition_complete callbacks were missing @key(fields: "id") directives and excluded from the _Entity union. In practice, this affected indexed types that are dynamically derived during schema finalization (e.g. aggregation types for location/merchant filtering).
• Regression from Schema support for inherited indexes #1067 ("Schema support for inherited indexes"): that PR moved @key directive addition into define_apollo_schema_elements, which runs as an early after_user_definition_complete callback. Types created by later callbacks don't exist yet when the @key logic iterates, so they silently lose their federation key.
• Fix: defer @key and _Entity logic into a nested after_user_definition_complete callback. Ruby's Array#each picks up elements appended during iteration, so the nested callback runs after all previously registered callbacks — ensuring all types exist first.

Test plan

• Added unit test that creates an indexed type in an after_user_definition_complete callback and verifies it gets @key(fields: "id") and is included in _Entity
• All 101 apollo unit tests pass
• Verified end-to-end with tf-sales-eg: schema_artifacts:dump produces no schema.graphql diff (previously OrderLocation and OrderMerchant lost their @key directives)

🤖 Generated with Claude Code

[myronmarston]

Left a few questions but LGTM overall.