Export: preserve Dtype state values in export callback arguments

[skryl]

Proposed changes

This PR fixes export callback state serialization for primitives that include Dtype in their state (for example AsType), so callback arguments preserve dtype values instead of dropping them.

Add explicit handling for Dtype during state unpacking:

else if constexpr (std::is_same_v<T, Dtype>) {
  unpacked_state.push_back(state);
}

It also adds a regression test to validate callback behavior and updates export docs with callback payload details.

Problem

In mlx/export.cpp, the state unpacking logic did not handle Dtype in tuple/variant flattening.
As a result, primitives with dtype state (for example AsType, NumberOfElements) could emit incomplete arguments during export.

Why this is needed

Downstream IR/export consumers rely on complete primitive arguments. Missing dtype state breaks robust graph export/lowering paths and forces downstream inference fallbacks.

Checklist

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[awni]

Thanks!