Add regression matrix for heterogeneous union annotations

[wbarnha]

Summary

• Add a focused compatibility suite for Faust model field unions.
• Cover both typing.Union and PEP 604 (X | Y) annotations.
• Preserve coverage for currently supported scalar, optional-model, and nested-container forms.
• Reproduce the exact declaration from NotImplementedError: Union of types not supported mode#80.
• Separate JSON-shaped pass-through candidates from ambiguous/coercion-sensitive unions that should remain explicitly unsupported.

Current finding

Mode 0.5.3 now correctly recognizes PEP 604 unions. Faust's UnionNode only collapses unions when every member is a model or every member is a literal scalar. A union such as str | list | dict | None therefore reaches UnionNode.build() and raises NotImplementedError during Record class creation.

The issue #80 cases are marked strict xfail in this draft so CI documents the existing failure while the implementation fix is designed. Ambiguous model/scalar and coercion-sensitive unions remain passing negative tests.