chore: use deptry optional_dependencies_dev_groups

[adamtheturtle]

Renames pep621_dev_dependency_groups to optional_dependencies_dev_groups for deptry 0.25+.

This removes deprecation warnings; the old key may be removed in a future deptry release.

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Note

Low Risk
Low risk configuration-only change: updates the deptry setting key to the new name to avoid deprecation warnings, with no runtime code impact.

Overview
Updates pyproject.toml deptry configuration to replace deprecated pep621_dev_dependency_groups with optional_dependencies_dev_groups while keeping the same dev/release groups, eliminating deptry 0.25+ deprecation warnings.

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