fix(gui): PluginMarketplace unhandled error — 'filter' of undefined in tests

[amiable-dev]

Description

When running npx vitest run, WorkspacePanel tests trigger an unhandled exception in PluginMarketplace:

TypeError: Cannot read properties of undefined (reading 'filter')
  in PluginMarketplace.svelte
  in PluginView.svelte
  in WorkspacePanel.svelte

This occurs twice (in two different WorkspacePanel test contexts). The root cause is that the invoke('get_plugin_registry') mock returns undefined instead of an object with iterable categories, so filteredPlugins calls .filter() on undefined.

Impact

• All tests still pass (711/711), but vitest reports it as an "Unhandled Error"
• Could mask real failures in the future

Expected Fix

Ensure PluginMarketplace handles missing/undefined plugin registry gracefully (default to empty array), and/or update the WorkspacePanel test mocks to return the expected shape.