Fix pylsp-rope compatibility with Jupyter Notebooks

[mcepl]

Implement guard clauses to handle virtual documents (notebook cells) safely:

• Add is_virtual_document() helper to detect non-file URIs
• Update get_resource() to return None for virtual documents
• Add early returns in pylsp_code_actions() and pylsp_rename() hooks
• Prevent rope operations on virtual documents that don't correspond to files

This fixes the python-lsp-server notebook test failure where pylsp-rope was interfering with notebook cell handling by trying to treat virtual URIs as filesystem paths.