model : fix SWA not being enabled for EXAONE 4.5 (#26848)

model : fix SWA not being enabled for EXAONE 4.5 (#26848)

* model : fix SWA not being enabled for EXAONE 4.5

load_arch_hparams tests `hparams.n_layer() == 64` before
LLM_KV_NEXTN_PREDICT_LAYERS has been read. n_layer() returns
n_layer_all - n_layer_nextn and n_layer_nextn defaults to 0, so a GGUF
carrying the MTP head (block_count=65, nextn=1) evaluates to 65 and the
whole SWA block is skipped. The model type switch further down in the
same function reads 64, because by then the key has been loaded.

n_swa is still filled in by the unconditional get_key below the block, so
llama_model_n_swa() reports 4096 and the logs look correct while only
swa_type stays LLAMA_SWA_TYPE_NONE.

This affects the official LGAI-EXAONE GGUF release as well. EXAONE 4.0 has
no MTP head, so block_count is 64 there and the check matches.

* model-loader : skip TENSOR_SKIP tensors in the metadata-only path

create_tensor asserts on a null buffer type when building from metadata
alone, but buft_for_tensor returns null by design for tensors marked
TENSOR_SKIP, which is how architectures with nextn/MTP layers mark theirs.
Those models cannot be constructed by llama_model_init_from_user at all.

The file-backed path below already returns nullptr for the same tensors, so
callers see the same thing either way.

* tests : cover exaone4 hparams ordering

Builds a synthetic exaone4 model with the layout the shipped EXAONE 4.5
GGUFs use (block_count 65 + nextn 1). The swa_type check is the one that
catches the ordering bug; the n_layer_nextn and n_layer() checks only tell
a broken fixture apart from a real regression.

Fails before the ordering fix with "swa_type is not STANDARD", passes after.

* Revert "tests : cover exaone4 hparams ordering"

This reverts commit d2f3bafeee591ad691396b2708de4baef3aaf602.

* Revert "model-loader : skip TENSOR_SKIP tensors in the metadata-only path"

This reverts commit aecb9bc0c7896b52afbc43921a1f572aa7b5e53c.