cuda: align snake fusion matcher with the other backends (#25460)

• cuda: fix snake fusion type predicate, a and inv_b are F32

The matcher required a->type == x->type while launch_snake reads both as const float *, matching the CPU and Metal contract where a and inv_b stay F32. F16/BF16 chains never fused and fell back to the naive path, and a hypothetical all F16 chain would have read F16 bits as float. Aligns the predicate and the comment with ggml-cpu.c

• cuda: reject snake fusion on non-contiguous operands

The kernel reads x[idx] and a[c] / inv_b[c] linearly, so a non-contiguous view passing the matcher would silently read wrong data. Mirror the contiguity guard already present in the CPU, Vulkan and Metal matchers.