fix: replace fixed sleeps with polling in flaky TestChildProcessCleanup tests

[BabyChrist666]

Summary

Closes #1775

The three tests in TestChildProcessCleanup (test_basic_child_process_cleanup, test_nested_process_tree, test_early_parent_exit) fail intermittently on macOS CI because fixed-duration anyio.sleep() calls (0.5s–1.0s) aren't always enough for child processes to start writing to their marker files on slow CI machines. The assertions then check assert 0 > 0 because the file is still empty.

Root cause

# Before: fixed sleep that may not be enough on slow CI
await anyio.sleep(0.5)
initial_size = os.path.getsize(marker_file)
await anyio.sleep(0.3)
size_after_wait = os.path.getsize(marker_file)
assert size_after_wait > initial_size  # fails with 0 > 0

Fix

Added a _wait_for_file_growth() helper that polls the marker file until it exists, has content, and is actively growing — with a generous 10-second timeout. This eliminates the race condition entirely:

# After: poll until file is growing, up to 10s
await _wait_for_file_growth(marker_file, "child process")

What changed

• Added _wait_for_file_growth() async helper (polls file size at 0.2s intervals, 10s timeout)
• Replaced all fixed sleep + assertion blocks in the 3 affected tests with the polling helper
• Net result: 35 insertions, 32 deletions — simpler and more reliable

Test plan

• ruff lint + format clean
• pyright clean (0 errors)
• Tests verified to work on CI (the original tests pass on CI already, this fix prevents the intermittent failures on macOS)

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