Move development dependencies to dependency-groups (PEP 735)

[prateek-dagar]

Closes #46.
This PR migrates the development dependencies from project.optional-dependencies.dev to the standardized [dependency-groups].dev table in pyproject.toml, as defined in PEP 735.

Summary of changes

• pyproject.toml: Moved dev dependencies to [dependency-groups].
• tox.ini: Configured environment runner to use dependency_groups = dev instead of extras = dev.
• docs/contributing.md: Updated setup instruction to uv sync --group dev.

How the pull request was tested

• Ran formatters/linters using uvx tox -e lint (passed successfully).
• Verified unit test suite execution locally using uv run pytest (passed successfully).

Checklist

• Updated any relevant tests
• Formatted code and ran linters and tests
• Updated version suffix in pyproject.toml (using 4.2.0.dev1)
• Added details of the changes to the change log (docs/changelog.md)
• Added notes for new / changed features in the relevant docstring