PartDesign, Sketcher, and Assembly toolbars use
DefaultVisibility::Unavailable and rely on EditingContextResolver to
show them. When switching to these workbenches without a specific editing
state (e.g. PartDesign with no Body activated), no context matches and
all workbench toolbars stay hidden.
Add workbench-level fallback contexts at priority 20 that match when the
workbench is active regardless of editing state:
- partdesign.workbench: shows Helper Features + Sketcher toolbars
- sketcher.workbench: shows Sketcher + Sketcher Tools
- assembly.workbench: shows Assembly toolbar
These fill the gap between object-specific contexts (priority 30+) and
the empty_document fallback (priority 10).
