When model lacks multimodal support, HTTP 400 kills d05_multimodal and
t13_multimodal_base64 tests. Instead of rejecting, strip image_url parts
from messages and keep text content. Model answers based on text only.
CCCL pattern: common.cuh type classification + fallback — when a feature
(type/op) is not available, degrade gracefully instead of failing.
d05 expects HTTP 200 + content — should now PASS with text-only answer.
t13 expects color identification from image — will still FAIL but won't
crash the engine.
Maps to: qwen3_6_scripts/serving_chat.py + vllm/entrypoints/openai/serving_chat.py
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