[roll] Roll fuchsia [starnix][tests] Refactor SubreaperCloneExitSignal to avoid race and globals

Refactors SubreaperCloneExitSignal to use Rendezvous for synchronization
and eliminate global variables.

Previously, the parent process (P1) could exit too quickly after writing
to the pipe, causing the child (P2) to be reparented to the subreaper
before it could assert that its parent was still P1 via getppid().

This change:

- Replaces global pipes with Rendezvous captured by reference in the
  clone lambdas.
- Synchronizes P2's getppid() check with P1's exit, ensuring P1 waits
  for P2 to finish the check before exiting.
- Uses SAFE_SYSCALL() and SyscallSucceeds().
- Inlines the parent and child helper functions as lambdas.

Test: starnix_syscalls_cpp_tests (waitpid_exit_signal_test)
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1638400
Original-Original-Revision: 76aa0bcbdafc96a13e1375850d52b9845486fed4
GitOrigin-RevId: fac91f2e806d7d06a947c8d3a620ac7a23594f3f
Change-Id: Ida738d3746239534048c3be03c60d7eb6b0f925b
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1639961
Cr-Commit-Position: refs/heads/main@{#194219}