Refactor and cleanup WorkspaceVariable handling

What does this MR do and why?

Refactoring and cleanup of WorkspaceVariable handling. See NOTEs in comments for details.

Related to Introduce gl_workspace_reconciled_actual_state ... (#520866 - closed)

References

• Introduce gl_workspace_reconciled_actual_state ... (#520866 - closed)
• Introduce gl_workspace_reconciled_actual_state ... (!182392 - merged)
• Startup scripts for Remote Development workspac... (&15602)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

• Create a workspace, open the terminal in VS Code, and ensure that all environment and file variables are still properly set.