[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dev/migrate-MR-v2 history (10 merge commits + superseded pre-rebase work)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>

examples/online_serving/qwen2_5_omni/run_curl_multimodal_generation.sh

@@ -1,6 +1,9 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+# Default model name
+MODEL="${MODEL:-Qwen/Qwen2.5-Omni-7B}"
+
 # Default query type
 QUERY_TYPE="${1:-mixed_modalities}"
 
@@ -162,7 +165,7 @@ echo ""
 
 request_body=$(cat <<EOF
 {
-  "model": "Qwen/Qwen2.5-Omni-7B",
+  "model": "$MODEL",
   "sampling_params_list": $sampling_params_list,
   "mm_processor_kwargs": $mm_processor_kwargs,
   "modalities": $MODALITIES,