Expand Pester coverage across install, tweaks, and UI workflows

[ChrisTitusTech]

Summary

• Adds and broadens Pester coverage for core install, uninstall, tweak, update, and runspace flows.
• Tightens validation around helper functions, package selection, logging, and UI state behavior.
• Updates workflow and project guidance to reflect the expanded test surface and current architecture.

Testing

• Expanded and updated Pester suites for the affected workflows and helpers.
• Workflow and documentation changes were validated alongside the test additions.
• Not run (not requested)