feat(mongodb): nested collection subtree in ConnectionsPanel

[ZhuchkaTriplesix]

Closes #1023

Changes

• Implemented lazy-loaded collection subtree under MongoDB database nodes in ConnectionsPanel
• Supported single-click navigation directly to MongoDocumentsView via WorkspacePanel
• Added inline quick filter for collection list with collection count pill
• Added context menu for collections: Open collection, Copy name, Refresh collections, Drop collection (with QueryaConfirmDialog)
• Added context menu for database: Copy name, Refresh, Drop database (with QueryaConfirmDialog)
• Updated QueryaSchemaObjectCache when MongoDB collections load
• Added unit and widget tests for workspace state and panel propagation