Improving Regression Test Coverage

Blender's UI regression test suite leaves most of the editor surface untested in CI, meaning regressions are caught only by manual review. This project extends the existing UI simulate testing framework with new targeted tests covering Window Manager state transitions, all practical editor/region types, menu and operator search per editor, tool/modifier/preference draw stability, and interaction-heavy flows like drag/drop, pie menus, and selection behavior. All tests integrate into the existing CI, with documentation covering how to run and extend UI regression tests within the framework.