fix: resolve import errors and test failures after segment 4 merge

- Update BaseNodeData import path to dify_graph.entities.base_node_data
- Change NodeType.COMMAND/FILE_UPLOAD to BuiltinNodeTypes constants
- Fix system_oauth_encryption -> system_encryption rename in commands
- Remove tests for deleted agent runner modules
- Fix Avatar: named import + string size API in collaboration files
- Add missing skill feature deps: @monaco-editor/react, react-arborist,
  @tanstack/react-virtual
- Fix frontend test mocks: add useUserProfile, useLeaderRestoreListener,
  next/navigation mock, and nodeOutputVars to expected payload

Made-with: Cursor