/claim #9909

This PR implements a comprehensive test suite for ZIOApp behavior as requested in issue #9909.

What was implemented:

• Complete test coverage for ZIOApp exit codes, finalizers, signals, timeouts, and regressions
• Process-based testing for real ZIOApp behavior verification
• Platform-appropriate handling with Windows signal test skipping
• CI-resistant timing with generous bounds to prevent flakes
• Regression tests for issues #9240, #9807, #10122

Production Bug Fix

While implementing the test for issue #9240 (custom exit codes), I discovered and fixed a bug in ZIOAppPlatformSpecific.scala. The exitWith method was not properly propagating custom ExitCode instances, always defaulting to ExitCode.success. This has been corrected to handle Exit.Success cases with custom exit codes.

Bonus: Flaky Test Fix

Fixed TestClockSpecJVM flaky test by adding ZIO.yieldNow after TestClock.adjust. The scheduled executor test was racy because adjust advances virtual time but doesn’t wait for async Runnables to complete.

Test Results

All tests pass locally:

• 2877 tests passed, 0 failed, 14 ignored
• Includes our new ZIOApp behavior tests