[MR V2] Migrate to Model Runner V2

Squashed migration rebased onto current main. Collapses the 46-commit
dev/migrate-MR-v2 history (10 merge commits + superseded pre-rebase work)
into a single commit, so the diff reflects only the genuine MR-V2 feature
(worker_v2/ runners + model_states, omni schedulers, stage engine pipeline,
pooling_output serialization bridge). Content already merged to main
separately (step_audio2, tts_adapters, npu/_310p, etc.) drops out.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/worker_v2/test_omni_model_state_plugin.py

@@ -0,0 +1,71 @@
+"""Unit tests for OmniModelStatePlugin interface correctness."""
+
+from types import SimpleNamespace
+from typing import Any
+
+import pytest
+import torch
+
+from vllm_omni.worker_v2.model_states.plugin import OmniModelStatePlugin
+
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
+
+class _ConcretePlugin(OmniModelStatePlugin):
+    """Minimal concrete plugin for testing the ABC."""
+
+    def __init__(self):
+        self.added: list[tuple[int, Any]] = []
+        self.removed: list[int] = []
+
+    def on_add_request(self, req_index, new_req_data):
+        self.added.append((req_index, new_req_data))
+
+    def on_remove_request(self, req_index):
+        self.removed.append(req_index)
+
+    def postprocess(self, text_hidden, multimodal_outputs, input_batch, req_states):
+        return text_hidden, multimodal_outputs
+
+
+def test_concrete_plugin_instantiates():
+    plugin = _ConcretePlugin()
+    assert isinstance(plugin, OmniModelStatePlugin)
+
+
+def test_on_add_request_called():
+    plugin = _ConcretePlugin()
+    plugin.on_add_request(0, SimpleNamespace(req_id="r1"))
+    assert len(plugin.added) == 1
+    assert plugin.added[0][0] == 0
+
+
+def test_on_remove_request_called():
+    plugin = _ConcretePlugin()
+    plugin.on_remove_request(3)
+    assert plugin.removed == [3]
+
+
+def test_prepare_extra_inputs_default_empty():
+    plugin = _ConcretePlugin()
+    result = plugin.prepare_extra_inputs(SimpleNamespace(), SimpleNamespace())
+    assert result == {}
+
+
+def test_dummy_run_default_noop():
+    plugin = _ConcretePlugin()
+    plugin.dummy_run(None, 128)
+
+
+def test_postprocess_returns_inputs():
+    plugin = _ConcretePlugin()
+    hidden = torch.randn(4, 8)
+    mm = {"audio": torch.randn(4, 2)}
+    out_h, out_mm = plugin.postprocess(hidden, mm, None, None)
+    assert out_h is hidden
+    assert out_mm is mm
+
+
+def test_abc_cannot_instantiate_directly():
+    with pytest.raises(TypeError):
+        OmniModelStatePlugin()