fix(tests): validate training completion in UT and capture torchrun exit code

[WangLingxun]

1. Shell fix (primus-cli-direct.sh):

   • Add set +e before eval "$CMD" so the script does not exit
     immediately on non-zero torchrun return code, allowing proper
     error logging before exit.

2. UT validation (tests/utils.py → run_training_script):

   • Extract common training-script execution logic into a shared
     'run_training_script()' helper, replacing duplicated code across
     test_megatron_trainer, test_torchtitan_trainer, and
     test_maxtext_trainer.
   • In the success path (exit code 0), assert that the PrimusRuntime
     'Training completed.' marker is present in the log file. This
     catches silent training failures where torchrun returns 0 but
     training did not actually finish (e.g. AITER HIP errors).