improve Dynamic validation

[matt-beanland]

Improve cast_input/apply stored. The input type for Value and Dynamic should perhaps be Unions. If we have the type dynamically we should apply constraints on it, so that i) it is a properly formed Value with Primitive or Dynamic and/or the Primitive/Dynamic is properly formed. The dynamic should be tested according to the type of the dynamic indicated. It should actually match the type, and fields should be valid.

Dynamic values are limited to Ash.Type.NewType that have storage_type: :map.
These are Ash.TypedStruct, or Ash.Type.NewType that are subtype_of keyword, tuple, map, union, struct

They shouldn't and probably can't be used for Elixir types that happen to be structs in Elixir, such as Decimal, Date, Time, DateTime, etc, these all have their own storage_type.

[matt-beanland]

iex(7)> Ash.Type.storage_type(Ash.Type.Keyword)
:map
iex(8)> Ash.Type.storage_type(Ash.Type.Tuple)
:map
iex(9)> Ash.Type.storage_type(Ash.Type.Map)
:map
iex(10)> Ash.Type.storage_type(Ash.Type.Union)
:map
iex(11)> Ash.Type.storage_type(Ash.Type.Struct)
🍁

[matt-beanland]

We need to check the module is loaded before despatching anything to it.

[matt-beanland]

Some work on nil safety for Diffo.Type.Value and Diffo.Type.Dynamic was done in 68-dynamic-nil-safety

[matt-beanland]

Problem: Dynamic accepts any atom as :type without checking it's actually usable. Three things can go wrong: the module isn't loaded, it's not a NewType, or it's a NewType but with storage_type other than :map (e.g. Ash.Type.Date, Ash.Type.Decimal).

Planned changes — Diffo.Type.Dynamic only:

Add valid_dynamic_type?/1 — private guard that checks all three conditions in order:

Code.ensure_loaded?(type) — module exists
Ash.Type.NewType.new_type?(type) — is a NewType
Ash.Type.storage_type(type, []) == :map — has map storage
Update cast_input — return {:error, reason} when type fails the guard, before attempting the cast

Update dynamic_constraints/1 — add the storage_type == :map check alongside the existing new_type? check, so it returns [] for scalar NewTypes

Update apply_constraints — validate the type field on the struct, currently accepts any %Dynamic{}

Doctests — add examples showing cast_input rejects Ash.Type.Date and an unloaded module

Unit tests — add to dynamic_test.exs: invalid NewType (wrong storage), unloaded module, valid map-storage type still works

No changes to Value or Primitive — Value's cast_input already delegates to Dynamic's cast_input for the dynamic member. The validation happens at the right level.

[matt-beanland]

done, also added Dynamic.is_valid? as part of this, surfaced to the API and covered in the livebook. Potentially would be useful as an ash expression.