Add ART Megatron support for GPT OSS

[FurtherAI]

Summary

Adds ART Megatron support for GPT OSS, including native vLLM LoRA serving, GPT OSS LoRA layout conversion/export, MXFP4 expert weight loading, sliding-window attention handling, and model-support workflow coverage.

Notable Non-Handler Changes

• Add handler-driven HF weight-source planning so GPT OSS MXFP4 grouped expert weights can be materialized through Megatron Bridge while preserving the default direct safetensors load path.
• Extend LoRA export/publish logic for GPT OSS packed expert layouts and explicit q/k/v target subsets.
• Add handler-provided vLLM runtime defaults for GPT OSS MoE backend and OpenAI tool parser, while preserving explicit user config.
• Implement Megatron softmax sink support in ART flex attention and CP attention/executor paths, plus the SM90 FlashAttention block-sparse dQ compatibility patch.
• Add sparse MoE route-replay metadata lower-bound handling so inferred num_experts is never lower than topk.
• Add model-support workflow ergonomics and diagnostics: single-stage runs, paired train-inf rerun guidance, and richer forward-trace hooks.
• Update length trainability for reasoning models: unbounded scenario stream, small per-completion max-token variation, GPT OSS chat-template kwargs/system prompt, 20-step budget, and GPT OSS-specific pass thresholds.
• Suppress Megatron-Core's expected DeepEP router-prob warning globally in ART provider setup; Megatron still casts bf16/fp16 router probs to fp32 before DeepEP dispatch.

Validation

• Full GPT OSS model-support workflow passed earlier, including train_inf_mismatch, correctness_sensitivity, length_trainability, and native_vllm_lora.
• Reviewed by a sub-agent; only low-severity test coverage nits were found and addressed.
• uv run --no-sync prek run ty --all-files
• uv run --no-sync prek run ruff --all-files
• uv run --no-sync prek run ruff-format --all-files
• uv run --no-sync prek run uv-lock-check --all-files
• uv run --no-sync pytest --nbval --current-env --tb=short tests/unit/test_moe_routing_real_path.py tests/integration/megatron/model_support/test_workflow.py::test_build_validation_report_only_stage_skips_other_stages