fix: remove unnecessary Pool(1) wrapper in _VwPy.run()

[JohnLangford]

Summary

Use spawn context for the Pool(1) in _VwPy.run() to enable compatibility with pybind11 bindings.

Problem

The Pool(1) wrapper is required because VW is not thread-safe internally - each VW call must run in its own subprocess for isolation.

However, the default fork-based Pool doesn't work with pybind11 bindings because pybind11 modules don't survive fork() cleanly, causing segfaults or hangs.

Solution

Use multiprocessing.get_context('spawn') to create a spawn-based Pool:

def run(self, args: str, filename=None) -> Iterable[str]:
    # Use spawn context to avoid fork issues with pybind11
    import multiprocessing
    ctx = multiprocessing.get_context('spawn')
    with ctx.Pool(1) as p:
        return p.apply(_run_pyvw, [args], {"filename": filename})

The spawn context starts fresh Python interpreters instead of forking, which is compatible with pybind11.

Testing

Verified locally that vw_executor works with pybind11-based vowpalwabbit bindings when called from Python scripts.

Note

Code that calls vw_executor at module import time (before if __name__ == '__main__' guard) may still have issues because spawn needs to re-import the main module in child processes. Such code should be restructured to defer VW execution to runtime.

Fixes #80