fix(computer-server): lazy-load native handlers

[spqw]

Summary

• Lazy-load native OS handlers only after the backend is selected.
• Avoid importing macOS native handlers when CUA_BACKEND=vnc / CUA_VNC_HOST is set.
• Prevent VNC-backed Lume/Tart sessions from triggering native macOS Accessibility/Screen Recording permission prompts unnecessarily.

Motivation

computer_server.handlers.factory imported the native macOS handler at module import time whenever the host OS was Darwin. The macOS handler intentionally probes Accessibility and Screen Recording permissions on import. For Lume VMs configured with the VNC backend, those native handlers are not used, but the import still causes the permission prompt.

This keeps the existing native behavior unchanged while making the VNC backend independent from native macOS handler side effects.

Testing

• python3 -m py_compile libs/python/computer-server/computer_server/handlers/factory.py
• Manually reproduced on a Lume Tahoe VM with CUA server configured for VNC backend (CUA_BACKEND=vnc, CUA_VNC_HOST, CUA_VNC_PORT): /status and /cmd screenshot remain functional through the VNC backend.

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