llama : add llama_model_ftype_name() (#25134)

llama : add llama_model_ftype_name() (#25134)

* llama : add llama_model_ftype_name()

Expose the model file type (quantization) name, e.g. "Q8_0" or
"Q4_K - Medium", through a new public C API. The returned pointer is
valid for the lifetime of the model and nullptr when the model is
invalid or the file type is unknown.

Signed-off-by: Adrien Gallouët <redacted>
* Export enum

Signed-off-by: Adrien Gallouët <redacted>
* s/llama_model_ftype_name/llama_ftype_name/

Signed-off-by: Adrien Gallouët <redacted>
* Move "(guessed)" to the front in llama_ftype_name

Prepend the "(guessed)" label instead of appending it. This allows removing
the non-thread-safe static std::string, making the function allocation-free.

Signed-off-by: Adrien Gallouët <redacted>
* Add LLAMA_FTYPE_PREFIX

Signed-off-by: Adrien Gallouët <redacted>
* Dont check for model

Signed-off-by: Adrien Gallouët <redacted>
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Signed-off-by: Adrien Gallouët <redacted>