[roll] Roll fuchsia [starnix] Pass Pid to set_foreground_process_group

Update ThreadGroup::set_foreground_process_group and
SessionMutableState::set_foreground_process_group to accept a &Pid
reference rather than primitive pid_t or &Arc<ProcessGroup>.

In devpts TIOCSPGRP, validate that the requested pgid is non-negative
and resolve it from the kernel's PidTable before calling
set_foreground_process_group. ThreadGroup::set_foreground_process_group
retrieves the process group directly via Pid::get_process_group()
while keeping the kernel.pids read lock to ensure atomicity against
concurrent process group updates or exits.

Multiply: starnix_core_tests
Test: fx test starnix_core_tests --test-filter "*devpts*"
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1810549
Original-Original-Revision: 66fb1fdd9b9ef5c90e74ae59188225f9d976f0a2
GitOrigin-RevId: e16d05ad45ac471d75838226b3f2917a330e63fe
Change-Id: Ic27c2255bdc7affebeaad70170f005776aa304ef
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1811989
Cr-Commit-Position: refs/heads/main@{#201490}