Fix msgspec tag_field conflict with discriminator field definition

[koxudaxi]

Summary

• Fix msgspec discriminator fields conflicting with tag_field parameter
• Skip rendering is_classvar fields in msgspec template (msgspec manages them via tag_field)
• Fix unused import removal to properly handle reference-counted imports

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.60%. Comparing base (747fed5) to head (7e5fa7f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2663      +/-   ##
==========================================
- Coverage   99.66%   99.60%   -0.06%     
==========================================
  Files          76       76              
  Lines       10481    10485       +4     
  Branches     1261     1263       +2     
==========================================
- Hits        10446    10444       -2     
- Misses         16       21       +5     
- Partials       19       20       +1     

Flag	Coverage Δ
unittests	99.60% <100.00%> (-0.06%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #2663 will not alter performance

_{Comparing fix/msgspec-discriminator-tag-field-conflict (7e5fa7f) with main (747fed5)}

Summary

✅ 50 untouched
⏩ 3 skipped¹

Footnotes

1. 3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩