[Bugfix][Spec Decode] Disable EAGLE prefill FULL CUDA graph for live multimodal batches

[luca-888]

Purpose

Fixes #43716.

EAGLE draft prefill FULL CUDA graph replay currently does not update per-batch multimodal embeddings/masks. For live multimodal batches, this can replay a graph without the current mm_inputs.

This PR disables only FULL CUDA graph mode for EAGLE draft prefill when the current batch contains live multimodal inputs. PIECEWISE remains allowed, and eager remains the fallback. Text-only batches, including text-only requests served by multimodal-capable models, keep the previous behavior.

The DP path also preserves downgrade decisions across ranks, so mixed DP batches cannot redispatch any rank back to FULL once one rank has downgraded to a lower CUDA graph mode.

Related work

• [Spec Decode][CUDA Graphs] Enables Eagle drafter support for FULL CUDA Graph mode #34880 and Support multimodal speculative decoding in draft_model mode #35714 touch nearby EAGLE / multimodal behavior, but they do not address this live multimodal EAGLE FULL prefill replay path.

Test Plan

The tests cover the dispatch policy for text-only batches, live multimodal batches, PIECEWISE/eager fallback, and DP downgrade preservation.

Test environment:

• Base commit: 158289e0

PYTHONPATH=$PWD python -m pytest tests/v1/cudagraph/test_gpu_cudagraph_utils.py -q

PYTHONPATH=$PWD ruff check \
  vllm/v1/worker/gpu/cudagraph_utils.py \
  vllm/v1/worker/gpu/dp_utils.py \
  vllm/v1/worker/gpu/spec_decode/eagle/speculator.py \
  tests/v1/cudagraph/test_gpu_cudagraph_utils.py

PYTHONPATH=$PWD ruff format --check \
  vllm/v1/worker/gpu/cudagraph_utils.py \
  vllm/v1/worker/gpu/dp_utils.py \
  vllm/v1/worker/gpu/spec_decode/eagle/speculator.py \
  tests/v1/cudagraph/test_gpu_cudagraph_utils.py

git diff --check

Test Result

6 passed, 16 warnings
All checks passed!
4 files already formatted

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @luca-888.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork