fix MacOS wait PID failure

[Guest0x0]

A recent refactor wrongly dropped the fix in #88. This PR re-introduce the fix in #88. Instead of perform a blocking wait, this PR uses small timeout + retry to work around the race window between kevent failing with ESRCH and the child process enter waitable state.

[coveralls]

Coverage Report for CI Build 592

Coverage decreased (-0.02%) to 77.134%

Details

• Coverage decreased (-0.02%) from the base build.
• Patch coverage: 5 uncovered changes across 1 file (8 of 13 lines covered, 61.54%).
• 1 coverage regression across 1 file.

Uncovered Changes

File	Changed	Covered	%
src/internal/event_loop/process_unix.mbt	5	0	0.0%
Total (3 files)	13	8	61.54%

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
src/internal/event_loop/process_unix.mbt	1	41.94%

---

Coverage Stats

Relevant Lines:	4627
Covered Lines:	3569
Line Coverage:	77.13%
Coverage Strength:	72904.66 hits per line

---

💛 - Coveralls