Implemented required_error from postgress to ash core

[dallingson]

Closes Issue in postgres:
ash-project/ash_postgres#261
More info can be found in the following PR:
ash-project/ash_postgres#707

Changes:

• Add core query function Ash.Query.Function.RequiredError implementing ash_required!/2, which returns the value when present or an Ash.Error.Changes.Required when it is nil.
• Add tests in test/query/required_error_test.exs covering nil/non‑nil values, string vs atom name keys, missing :resource / :name, new/1 argument validation, and metadata helpers (can_return_nil?/1, evaluate_nil_inputs?/0).
• Document the function with a short entry in documentation/topics/reference/expressions.md describing typical usage: expr(ash_required!(^value, ^attribute)) where attribute has at least :name and :resource.

Contributor checklist

Leave anything that you believe does not apply unchecked.

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• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

[zachdaniel]

Sorry I keep requesting changes on you! I think the function inside of Ash core itself should just be called required!. The one that ash_postgres defines inside your database should be called ash_required to match how we use the ash_ prefix elsewhere.

[dallingson]

im just seeing this comment now... I will work on this right now

[zachdaniel]

What we would then want to do is modify the code in Ash.Changeset that adds atomic required errors to use this function if the data layer supports it. We can use a capability like :required_error and check the data layer if it supports that capability.

[dallingson]

Did I get this implemented correctly?

[zachdaniel]

Looks like some failing checks to address 😄

[zachdaniel]

Also worth checking (or having claude check) if there are any other expression patterns that use that if is_nil(...) that should also use required when the data layer supports it 😄

[dallingson]

I fixed the formatting issues, and I used Claude to check to see if anything else should use required. As far as I can tell, I think we are good. Is there anything else I need to do for this issue?

[zachdaniel]

🚀 Thank you for your contribution! 🚀