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