chore: use deptry optional_dependencies_dev_groups

[adamtheturtle]

Renames pep621_dev_dependency_groups to optional_dependencies_dev_groups for deptry 0.25+.

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Note

Low Risk
Low risk: a config-only rename affecting how deptry interprets dev dependency groups, with no runtime code changes.

Overview
Updates pyproject.toml to rename deptry’s dev dependency groups setting from pep621_dev_dependency_groups to optional_dependencies_dev_groups, aligning the project with deptry 0.25+.

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