Make dist_opt validation model-agnostic

[ISEEKYAN]

Summary

• Remove the dist_opt optimizer primitive model-name allowlist so validation is model-agnostic.
• Keep the vpp/pp parallelism guard and update user-facing errors to use dist_opt terminology.
• Add unit coverage for custom model names, invalid vpp/pp config, and the validate_mc_session compatibility alias.

Tests

• PYTHONPATH=experimental/lite pytest experimental/lite/tests/unit/primitive/test_dist_opt_validation.py