fix: Handle sliced list arrays correctly in Avro writer

[ty1260]

Summary

• skip child values before the first logical offset when serializing sliced list arrays to Avro
• add a regression test covering sliced required and nullable list[str] columns in Avro roundtrip

Alternative to #27137.
Fixes #27090.

Test plan

• cargo test -p polars-arrow --features io_avro --lib

[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.83%. Comparing base (880651f) to head (0865c86).

Files with missing lines	Patch %	Lines
crates/polars-arrow/src/io/avro/write/serialize.rs	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27254      +/-   ##
==========================================
+ Coverage   81.58%   81.83%   +0.24%     
==========================================
  Files        1820     1820              
  Lines      251036   251045       +9     
  Branches     3149     3149              
==========================================
+ Hits       204808   205436     +628     
+ Misses      45420    44801     -619     
  Partials      808      808              

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