[Bug]: Qwen3.5 (hybrid GDN) models cannot run dflash speculative decoding with pipeline parallelism (missing supports_aux_hidden_states_over_pp opt-in)

Finding: In an external report on vLLM 0.30.0, serving a Qwen3_5ForConditionalGeneration (hybrid GDN) model with pipeline-parallel-size 2 and dflash speculative decoding fails at startup with a ValueError saying the model does not support dflash with pipeline parallelism. The reporter used heterogeneous RTX 3090 + RTX A4000 GPUs, the mp executor, the FLASHINFER attention backend and an fp8 KV cache (https://github.com/vllm-project/vllm/issues/60765). Reported cause (hypothesis, not maintainer-confirmed): the reporter says the startup check verify_supports_aux_hidden_states_over_pp() in eagle3_utils.py requires the inner decoder to declare supports_aux_hidden_states_over_pp. They state that only llama.py and qwen2.py set this flag, so Qwen3.5 is refused even though generic PP spec-decode transport (#50514) exists (https://github.com/vllm-project/vllm/issues/60765). The reporter also says Qwen3_5MTP with PP2 works on the same hardware. Additional contributor analysis: a contributor who reviewed main says adding the flag alone looks insufficient. They say Qwen3NextModel.forward captures stage-local auxiliary states but does not pack them into IntermediateTensors on non-final stages or collect upstream ones on the final stage. They also say the text-only Qwen3_5ForCausalLMBase.set_aux_hidden_state_layers() assigns the layer tuple directly rather than using EagleModelMixin._set_aux_hidden_state_layers() (https://github.com/vllm-project/vllm/issues/60765#issuecomment-6074450501). This is code-reading by a contributor, not verified here. Fix status: the contributor later reported an implementation and focused CPU tests on a personal branch at a specific commit, and asked the reporter to validate it on the PP2 setup before opening a draft PR (https://github.com/vllm-project/vllm/issues/60765#issuecomment-6076933022). As of the supplied content there is no merged fix, no released fix, and no validation results. The issue is open with no maintainer triage. Practical implication: if you hit this exact error with Qwen3.5 + dflash + PP, the sources describe it as a missing model opt-in and possibly missing stage-relay logic, with no documented workaround. Running without PP or without dflash is not documented in the sources as a workaround, so none is claimed. Watch the issue for a PR and release. Limitations: this is an external report; KFA did not reproduce it. The contributor branch was not reviewed or tested, and the affected-version scope beyond 0.30.0 is unknown.