[vm/io] Reap exit-handler wakeup process

[bsutton]

ExitCodeHandler::TerminateExitCodeThread() forks a short-lived child to
wake the exit-handler thread from wait(). If that thread observes
running_ == false before entering wait(), it exits without reaping the
wakeup child. In a container, that child can be adopted by a Dart process
running as PID 1 and remain as a zombie.

Retain the wakeup PID and call waitpid() after the exit-handler thread has
terminated. ECHILD is accepted because it means the handler thread already
reaped the process. Apply the equivalent change on Linux and macOS.

Add a Linux regression test that:

• makes the test process a child subreaper;
• invokes an activated executable through dart pub global run;
• verifies that no zombie child is adopted after the intermediate Dart VM
  exits.

Validation:

• Before the patch, the regression test reproduced the zombie on its first
  invocation using a source-built Dart 3.12.2 SDK.
• Before the patch, the container harness reproduced
  dart:dartdev_aot <defunct> under container PID 1.
• After the patch, the official standalone regression test passes.
• After the patch, the exact fatal-dartdoc container harness completed 30
  iterations without a zombie on Dart 3.12.2 and a source build from main.
• The focused standalone process suite passed 33 tests. The existing
  process_child_test could not create its process stream in the local
  restricted sandbox and failed independently of this change.

Fixes #61624.

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