graph : create V as a view of K in the k_iswa build_attn (#27392)

build_attn with the llm_graph_input_attn_k_iswa input was using the cached K
tensor itself as V. Create V as a view of K (the first v_cur->ne[0] elements
of each row), like the other K-only build_attn overloads.

The deepseek4 MTP call site now passes the kv tensor as v_cur.

Assisted-by: pi:llama.cpp/Qwen3.8-27B