refactor(vllm-omni/hi3): reuse FusedMultimodalCondition.concat for DiT capture padding

[chi030303]

Summary

hi3_fused_conditions right-padded per-request fused_mm_capture dicts with its own ragged-L helpers, and that copy still filled position_ids with 0. Those ids are upstream HunyuanImage3's KV-cache write slots, so a constant 0 pad writes every pad position of a short row onto cache slot 0.

HunyuanImage3FusedMultimodalCondition.concat already pads position_ids with each row's continuing indices (#479). This deletes the adapter's _pad_to / _pad_attn_mask assembly and calls that concat. hi3_ar_fused_conditions is unchanged.

Related Issue

Fixes #525

Test Plan

CPU check of hi3_fused_conditions against the previous padding, on three ragged captures (L = 7 / 11 / 9, 2 rows each) and on two equal-length captures:

• input_ids, attention_mask, gen_image_mask, and gen_timestep_scatter_index match the previous 0-pad assembly
• the short row's position_ids pad is [7, 8, 9, 10] instead of [0, 0, 0, 0]; the length-9 row's pad is [9, 10]
• equal lengths leave position_ids as 0..4
• a missing fused_mm_capture still raises the existing RuntimeError

Not run: pre-commit; reason: the hook runner is not installed in this environment.
Not run: GPU rollout or diffusion replay; reason: train-side replay calls predict_noise without a KV cache, so this position_ids change has no numeric effect on the current path. The zero pad becomes a real KV overwrite only on a cached forward.

Compatibility / Risk

No config, checkpoint, or API change. Ragged DiT captures now carry continuing position_ids instead of a 0 pad. Callers still receive {"fused": HunyuanImage3FusedMultimodalCondition}.

Reviewer Notes

The deleted helpers had no other callers. Hi3ImageOutputAdapter and Hi3DitRecaptionOutputAdapter only pass the returned dict through. An independent read of the diff found no blocking issues.

AI-assisted implementation.

Checklist

• I reviewed the changed code and removed unrelated/generated artifacts.
• I updated tests, docs, and configs where needed, or explained why not.

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