test: make parallel serdes assertions order independent

[Genmin]

Summary

• key parallel serialize/deserialize mock assertions by operation_id instead of observed call order
• keep checks for child item serdes fallback and parent batch serdes

Fixes #366

Validation

• uvx hatch test tests/operation/parallel_test.py -k "parallel_item"
• uvx hatch test tests/operation/parallel_test.py
• uvx hatch fmt --check
• uvx hatch run types:check tests/operation/parallel_test.py
• git diff --check

[yaythomas]

thank you so much @Genmin, great contribution, welcome to the repo! 🙇