Add required!/1, ash_required/1, and ash_required!/2 (issue #261)

[dallingson]

Pull request: Add required!/1, ash_required/1, ash_required!/2, and required_error/2

Closes #261

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What

Adds custom expressions required!/1 and ash_required/1 for presence in queries, and ash_required!/2 plus required_error/2 for required-field validation that returns Ash.Error.Changes.Required when a value is nil.

Why

There was no built-in way to express "required/present" in filters and calculations without not is_nil(...), or to get Ash's standard required error from the data layer when a value is nil.

How to use

Presence in queries (filters/calculations):

# Instead of: not is_nil(post[:title])
filter(ash_required(:title))

# Or with required!/1 (same meaning)
filter(required!(:title))

Required-field validation (returns Ash.Error.Changes.Required when nil):

change fn changeset, _context ->
  Ash.Changeset.require_change(changeset, :title, &AshPostgres.Functions.RequiredError.required_error/2)
end

When the value is nil, Ash uses required_error/2 and the data layer returns the standard required error.

Changes

• required.ex / ash_required.ex – Custom expressions required!/1 and ash_required/1 for presence in queries.
• required_error.ex – required_error/2 and data-layer integration so validation returns Ash.Error.Changes.Required when nil.
• ash_required.ex (expressions) – ash_required!/2 expression that uses the required-error path when the value is nil.
• data_layer.ex – can?(:required_error) and registration of the new functions/expressions.
• ash_functions.ex – PostgreSQL ash_required/1 in migrations so it's available in SQL.
• config.exs – Registered the new custom expressions.
• Docs – New content in documentation/topics/advanced/expressions.md.
• Tests – Filter, aggregate, calculation, and required_error_test.exs coverage.

Contributor checklist

Leave anything that you believe does not apply unchecked.

• 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

[zachdaniel]

I don't see any particular value in required, and ash_required. Fundamentally we just need the required error logic.