When SIGTERM is received, the dispose method was called but the process
did not exit, causing rstest to wait indefinitely and eventually kill
the process with SIGKILL, preventing proper test result collection.

Changes:
- Add shouldExit parameter to dispose method (default: false)
- Call process.exit(0) after cleanup when shouldExit is true
- Update signal handlers (SIGTERM, SIGINT, unhandledRejection, uncaughtException) to pass shouldExit: true
- Keep exit event handler with shouldExit: false to avoid double exit
- Manual dispose() calls in tests remain unaffected (default shouldExit: false)

This ensures that when rstest sends SIGTERM, the process properly exits
after cleanup, allowing rstest to collect test results normally.

Address review feedback: unhandledRejection and uncaughtException should
exit with non-zero status code (1) instead of 0, so CI/rstest can properly
detect failures and collect error information.

Changes:
- Change dispose parameter from shouldExit: boolean to exitCode?: number
- SIGTERM and SIGINT exit with code 0 (normal termination)
- unhandledRejection and uncaughtException exit with code 1 (error)
- exit event handler doesn't pass exitCode (process already exiting)