Improve metadata debug functionality with better field handling

[Kyle-Ye]

Summary

• Cleaned up metadata debug code by removing obsolete comments
• Improved field enumeration for structs and classes using .continueAfterUnknownField
• Added comprehensive test coverage for classes with unknown fields
• Enhanced code readability with proper hex offset formatting

Test plan

• Existing metadata debug tests pass
• New test for unknown fields handling
• Code builds successfully

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 4.81%. Comparing base (989275d) to head (b767453).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...urces/OpenAttributeGraphShims/Metadata+Debug.swift	0.00%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #182   +/-   ##
=====================================
  Coverage   4.81%   4.81%           
=====================================
  Files         28      28           
  Lines        831     831           
=====================================
  Hits          40      40           
  Misses       791     791           

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