Preserve nonzero exit status through fast-shutdown close

[hujc7]

Description

SimulationApp.close(exit_code) currently behaves differently depending on the exit code: a nonzero code exits immediately via os._exit before any cleanup runs, while the graceful path always terminates the process with status 0 from inside app.shutdown() under fast shutdown. Callers therefore have to choose between cleanup and a truthful exit status. In practice, a worker that closes gracefully after a failure or a termination signal is reported as exit 0, which distributed launchers (torchrun) and CI record as a successful run — the surviving ranks then block until the NCCL watchdog fires, and the root cause is misattributed. (Downstream context: isaac-sim/IsaacLab#6636 currently works around this by disabling fast shutdown at signal time.)

This change makes exit_code orthogonal to teardown:

• every close() path performs its normal cleanup regardless of the exit code;
• a nonzero status is applied with os._exit(exit_code) exactly at the point where fast shutdown would otherwise terminate the process with 0 (before _arm_shutdown_watchdog(), since that path never reaches app.shutdown());
• _flush_stdio() additionally flushes the C stdio streams (fflush(NULL) via ctypes): native output such as IApp.print_and_log is buffered on the C/C++ side, which os._exit abandons, so the tail of the shutdown log — including the Simulation App Shutting Down marker — was previously lost on every _exit path.

Verification

On a Linux source build (6.0.0 GA base, change rebased onto main with the shutdown-watchdog resolution described above), with a real headless app:

• SimulationApp({"headless": True}).close(exit_code=5) → process exits 5, full cleanup runs, and Simulation App Shutting Down is present in the output (previously: either exit 5 with no cleanup, or cleanup with exit 0).
• close() with the default exit code → unchanged: exits 0 with full cleanup.
• Isaac Lab's AppLauncher exit-status integration tests (SIGTERM status, unhandled-exception status) pass unchanged against the patched build.