Move from fork to forkserver as default multiprocessing context

[sjmonson]

Is your feature request related to a problem? Please describe.

GuideLLM defaults to fork as the multiprocessing context for workers. This is considered unsafe for Python applications that contain threads.

Describe the solution you'd like

We should switch to forkserver as the default multiprocessing method; as of Python 3.14 it is the default for MacOS and Linux.

Additional context

There are a few issues that must be addressed before the default can be changed:

1. Most multi-processing primitives are created in the default context rather then respecting GUIDELLM__MP_CONTEXT_TYPE.
2. GuideLLM is designed to explicitly import itself and all dependencies on import. This leads to slow process startups on Linux; due mostly to transformers and torch