fix: pydantic 2.12 compatibility.

[dapper91]

Due to pydantic 2.12 breaking changes FieldInfo can not be inherited anymore, FieldInfo.merge_field_infos is depricated.

The fix get rid of XmlEntityInfo inheritance from FieldInfo and uses FieldInfo metadata as an xml meta-information storage.

Fixes the issue #293 and #292

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 98.59155% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.27%. Comparing base (3fecbed) to head (3079a85).

Files with missing lines	Patch %	Lines
pydantic_xml/compat.py	87.50%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #295      +/-   ##
==========================================
- Coverage   92.31%   92.27%   -0.04%     
==========================================
  Files          30       31       +1     
  Lines        1730     1748      +18     
==========================================
+ Hits         1597     1613      +16     
- Misses        133      135       +2     

Flag	Coverage Δ
unittests	92.27% <98.59%> (-0.04%)	⬇️

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