fix: resolve TypeAliasType before issubclass in pack_union (fixes #330)

[gaoflow]

PEP 695 type aliases nested inside unions crash serializer generation
because pack_union passes TypeAliasType objects directly to
issubclass(get_type_origin(...), Collection).

On Python 3.13+, get_type_origin returns None for TypeAliasType,
and issubclass() fails. On 3.12, it returns the alias itself, which
also fails the check.

This adds a helper that recursively unwraps TypeAliasType via
\_\_value\_\_ before the issubclass calls, fixing both the eager
and lazy compilation paths.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.61%. Comparing base (22cf576) to head (64923fe).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
- Coverage   99.92%   99.61%   -0.31%     
==========================================
  Files         101      101              
  Lines       12038    12050      +12     
==========================================
- Hits        12029    12004      -25     
- Misses          9       46      +37     

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