metal : implement rope_back operator (#24725)

Reuse existing rope kernels with a function constant to toggle forward/backward
rotation, avoiding duplicate kernel code.

Assisted-by: pi:llama.cpp/Qwen3.6-27B

ggml/src/ggml-metal/ggml-metal-ops.cpp

@@ -375,6 +375,7 @@ static int ggml_metal_op_encode_impl(ggml_metal_op_t ctx, int idx) {
                 n_fuse = ggml_metal_op_norm(ctx, idx);
             } break;
         case GGML_OP_ROPE:
+        case GGML_OP_ROPE_BACK:
             {
                 n_fuse = ggml_metal_op_rope(ctx, idx);
             } break;