Handle nonexistent or inaccessible startup cwd gracefully in wspbus - #2087

Fixes #2055

Summary

When cherrypy is imported in an environment where the current working directory was removed or is inaccessible, wspbus.py crashed during module import because _startup_cwd = os.getcwd() was called unguarded at top level.

This change:

1. Catches (FileNotFoundError, OSError) when initializing _startup_cwd at import time, falling back to None.
2. Guards os.chdir(_startup_cwd) in _do_execv so that if _startup_cwd is None or raises (FileNotFoundError, OSError), it does not prevent process restart or crash.
3. Adds tests in cherrypy/test/test_bus.py verifying that importing with a nonexistent working directory succeeds and that _do_execv handles _startup_cwd=None and inaccessible directories gracefully.