ggml : add get_alloc_size_n to buffer type interface
- Add ggml_backend_buft_get_alloc_size_n public API
- Add optional get_alloc_size_n callback to ggml_backend_buffer_type_i
- Share tensor->buffer planning between alloc_buffer_n default and get_alloc_size_n default
- Replace unchecked realloc with std::vector in alloc_buffer_n default
- Make ggml_backend_alloc_ctx_tensors_from_buft_size use the new API
- Add test-alloc coverage for get_alloc_size_n
Assisted-by: pi:llama.cpp/DeepSeek-V4-Flash-Vision-Exp
