test(pp): add qwen3_5_moe PP and EP parity tests [3/4] (#3649)

Qwen3.5-MoE recipes ship pp_size 2-8 and ep_size 4-32, and nothing checked that
splitting the pipeline or sharding the experts leaves the numbers unchanged.

Adds a small proxy config plus two tests that run it twice and compare the loss
and gradient-norm curves: PP2 against a single-rank baseline, and EP2 against a
2-rank data-parallel run.

The shipped recipes are VLM, but no Qwen3.5 processor is staged in CI, so the
proxy uses the text-only Qwen3_5MoeForCausalLM and generates its own tokens.
Nothing has to be staged to run these.

Signed-off-by: Abhishree <abhishreetm@gmail.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>