Use dependency groups in openpi-client

[kstonekuan]

Summary

This updates packages/openpi-client/pyproject.toml to use dependency-groups.dev instead of the deprecated tool.uv.dev-dependencies field.

Why

uv now warns that tool.uv.dev-dependencies is deprecated and will be removed in a future release.

warning: The `tool.uv.dev-dependencies` field (used in `/Users/kkuan/openpi/openpi/packages/openpi-client/pyproject.toml`) is deprecated and will be removed in a future release; use `dependency-groups.dev` instead

Keeping the package on the deprecated field adds noise to routine uv commands and risks a future break once support is removed.

Validation

• ran uv lock --project packages/openpi-client --check --no-cache
• ran uv lock --project packages/openpi-client --dry-run --no-cache
• verified there are no lockfile changes

Notes

This is a configuration-only change and does not alter runtime dependencies or package behavior.