Adds left connector class to the rendered graph

What does this MR do?

Adds missing left-connector and left-margin classes to the pipeline graph. Forgot them while rewriting it in vue.js 😭

Screenshots (if relevant)

Before	After

Does this MR meet the acceptance criteria?

• Tests
  • Added tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #31053 (moved)