Rework on multiprocessing `fork` (RDT-792)

[hfudev]

Changed in version 3.12: If Python is able to detect that your process has multiple threads, the os.fork() function that this start method calls internally will raise a DeprecationWarning. Use a different start method. See the os.fork() documentation for further explanation.

Note The default start method will change away from fork in Python 3.14. Code that requires fork should explicitly specify that via get_context() or set_start_method().

Rework on the current process + thread solution, need to be tested on linux, macos, and windows.