[Refactor]: clean up diffusion scheduler interfaces (#5216)

Signed-off-by: Huang, Zeyu <11222265+fhfuih@users.noreply.github.com>
Signed-off-by: Zeyu Huang | 黃澤宇 <11222265+fhfuih@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

docs/design/feature/diffusion_step_execution.md

@@ -49,7 +49,7 @@ state object:
 
 The state lives in
 [`vllm_omni/diffusion/worker/utils.py`](gh-file:vllm_omni/diffusion/worker/utils.py)
-as `DiffusionRequestState`. Store request-scoped tensors there, or use
+as `StepRequestState`. Store request-scoped tensors there, or use
 `state.extra` for model-specific fields that do not justify extending the core
 dataclass.