metal : add f16 and bf16 support for concat operator (#24724)

metal : add f16 and bf16 support for concat operator (#24724)

* metal : add f16 and bf16 support for concat operator

Extend the Metal backend concat operator to support f16 and bf16 tensor
types in addition to the existing f32 and i32 support.

- Template kernel_concat on type T with specializations for float, half,
  bfloat, and int
- Add type-specific pipeline getter ggml_metal_library_get_pipeline_concat()
- Update device support check to allow f16 unconditionally and bf16 when
  device supports bfloat16
- Update dispatch to select the correct kernel specialization by type

Assisted-by: pi:llama.cpp/Qwen3.6-27B
* metal : extend concat operator to support f16, bf16, i8, i16 and i64

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