Validate Windows PE binary extension content beyond pe_type

[christopher-stoppa]

Summary

• Enforce the STIX 2.1 requirement that a Windows PE Binary File Extension contain at least one defined property besides the required pe_type.
• Preserve support for custom properties while ensuring they do not satisfy this extension-specific constraint.
• Update valid fixtures and add direct and nested File regression tests.

Fixes #618.

Validation

• python -m pytest stix2/test -q — 2519 passed, 3 xfailed, 1 xpassed.
• pre-commit run --all-files — all hooks passed.
• git diff --check — passed.

Reference

• OASIS STIX 2.1 specification, Windows PE Binary File Extension section.

[CLAassistant]

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