test(plugin): StatusButtonGroupBuilder workflow fallback tests (#2416)

[kitelev]

Summary

• Add 19 unit tests for StatusButtonGroupBuilder workflow fallback logic
• Cover standard class buttons (ems__Task, ems__Project), custom class workflow fallback, edge cases, and button properties
• Include StatusButtonGroupBuilder.ts with workflow fallback build() method and buildWorkflowButtons() private method
• Export WorkflowResolver and VisibilityGenerator from core package

Closes #2416