Stabilize RMCP pid file cleanup test

[aibrahim-oai]

What changed

• The pid-file cleanup test now keeps polling when the pid file exists but is still empty.
• Assertions only proceed once the wrapper has actually written the child pid.

Why this fixes the flake

• File creation and pid writing are not atomic as one logical action from the test’s point of view.
• The previous test sometimes won the race and read the file in the tiny window after creation but before the pid bytes were flushed.
• Treating “empty file” as “not ready yet” synchronizes the test on the real event we need: the wrapper has finished publishing the child pid.

Scope

• Test-only change.