Standardize 2D PCA output figures across MOSuite function calls

[phoman14]

Summary

This PR aligns the 2D PCA capsule with the package-level figure standardization work. The intent is for 2D PCA figures produced from this capsule to match figures produced by plot_pca_2d() and by MOSuite workflow functions that emit 2D PCA plots.

Changes

• Use the same default color palette as the MOSuite package for 2D PCA outputs.
• Document that random colors are used only as a fallback when the selected or provided palette has too few colors.
• Keep capsule parameter descriptions focused on the user-facing defaults.
• Update CHANGELOG.md for the user-facing default changes.

Validation: local capsule tests passed (PASS 11).

Issues

No linked issue.

PR Checklist

(Strikethrough any points that are not applicable.)

• This comment contains a description of changes with justifications, with any relevant issues linked.
• Run the capsule in Code Ocean and ensure it completes successfully with no errors. Local capsule tests passed.
• Update CHANGELOG.md with a short description of any user-facing changes and reference the PR number. Follow the style described in https://style.tidyverse.org/news.html
• The tests workflow succeeds on the latest commit.