Returns a fresh createMockTextEditor fixture. When document
looks like a mock TextDocument (has getText), it's threaded through
so result.document is the same object the caller passed in, matching
the real API's identity guarantee.
Test hook: switches the active theme and notifies
onDidChangeActiveColorTheme listeners, as VS Code does when the user
picks a different theme.
Darkby default. A plain mutable field, so a test can assign it directly — use_setColorThemeinstead when listeners registered throughonDidChangeActiveColorThemealso need to fire.