feat: add MLX distributed E2E test for MPI workload

[Rajkaran-122]

This commit adds comprehensive E2E testing for the MLX distributed runtime to complement the existing DeepSpeed MPI test. The MLX runtime uses OpenMPI for distributed training and requires verification of the launcher/node dependency relationship.

Changes:

• Add mlxRuntime constant to E2E test suite
• Add MLX workload test case with multi-node MPI configuration
• Verify launcher dependsOn node status (Ready) for proper MPI bootstrap
• Test validates both launcher and node job completion status

This ensures the MLX distributed runtime works correctly with the MPI plugin and JobSet dependsOn API, providing parity with DeepSpeed runtime testing.

Co-Authored-By: yadavrajkaran854@gmail.com

Fixes #3174

[github-actions]

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