[Diffusion] Prepare HunyuanImage3 for Scheduler-managed paged KV cache (#5541)

Signed-off-by: zwhzzz0821 <zwhzzz0821@users.noreply.github.com>
Co-authored-by: zwhzzz0821 <zwhzzz0821@users.noreply.github.com>
Co-authored-by: Hongsheng Liu <liuhongsheng4@huawei.com>

vllm_omni/diffusion/diffusion_kv/config.py

@@ -0,0 +1,39 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from __future__ import annotations
+
+from enum import Enum
+
+
+class DiffusionKVCacheMode(str, Enum):
+    """Migration mode for diffusion KV ownership."""
+
+    DENSE_LEGACY = "dense_legacy"
+    # Reserved for the experimental Worker-owned paging design. Production
+    # configuration rejects it until that engine is migrated to the common
+    # Scheduler-owned path.
+    PAGED_WORKER_LOCAL = "paged_worker_local"
+    PAGED_SCHEDULER = "paged_scheduler"
+
+
+def parse_diffusion_kv_cache_mode(value: object) -> DiffusionKVCacheMode:
+    """Parse a selectable Diffusion KV ownership mode.
+
+    ``PAGED_WORKER_LOCAL`` remains in the enum to document the migration
+    topology, but it is not a valid production selection until that runtime is
+    wired. Keep this validation shared by structured and runtime configs so an
+    unsupported mode fails before managed-process startup.
+    """
+
+    try:
+        mode = DiffusionKVCacheMode(value)
+    except (TypeError, ValueError) as exc:
+        supported = ", ".join(mode.value for mode in DiffusionKVCacheMode)
+        raise ValueError(f"Unsupported Diffusion KV diffusion_kv_mode {value!r}; expected one of: {supported}") from exc
+    if mode is DiffusionKVCacheMode.PAGED_WORKER_LOCAL:
+        raise ValueError(
+            "Diffusion KV diffusion_kv_mode 'paged_worker_local' is reserved but not implemented; "
+            "use 'dense_legacy' or 'paged_scheduler'"
+        )
+    return mode