[Model] Support stable diffusion3

[iwzbi]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

Support stable diffusion3 #311

Test Plan

Test Result

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[chatgpt-codex-connector]

Fix malformed diffusion model registry entry

The _DIFFUSION_MODELS dictionary is syntactically invalid because the LongCatImageEditPipeline entry is missing its closing parenthesis and trailing comma before the StableDiffusion3Pipeline entry, leaving the file unparsable. Importing vllm_omni.diffusion.registry will raise a SyntaxError, preventing any diffusion model (including the new SD3 pipeline) from being registered or loaded.

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