preexec_fn=os.setsid runs Python code in the forked child before exec,
which is unsafe in multi-threaded processes (CPython docs). When the
Desktop gateway loads native libraries (onnxruntime, BLAS, provider SDKs)
with active thread pools, the fork can SIGSEGV before the child execs.
Replace all preexec_fn usage with start_new_session=True, which provides
the same setsid/process-group semantics without running Python in the
fork. This is already the pattern used throughout hermes_cli/gateway.py
and hermes_cli/_subprocess_compat.py.
Fixes#46789
515192c4b9