llama : fix pooling assertion crash in chunked GDN detection path (#20468)

llama : fix pooling assertion crash in chunked GDN detection path (#20468)

* llama : fix pooling assertion crash in chunked GDN detection path

The chunked fused Gated Delta Net detection in sched_reserve() calls
graph_reserve(16*n_seqs, n_seqs, n_outputs, ...) where n_outputs = n_seqs.
This creates a dimension mismatch in build_pooling() for embedding models
with mean/rank pooling: build_inp_mean() creates a tensor with shape
[n_tokens=16*n_seqs, ...] while t_embd is reduced to [n_outputs=n_seqs, ...]
via out_ids, causing ggml_mul_mat to assert on ggml_can_mul_mat(a, b).

Fix: pass n_tokens as n_outputs in the chunked GDN graph reservation,
matching the pattern used by the pp/tg worst-case reservations.

Regression introduced by #20340 (d28961d).
Same class of bug as #12517, fixed by #12545.

* server : add mean pooling tests to embedding test suite

Add test_embedding_pooling_mean and test_embedding_pooling_mean_multiple
to cover the --pooling mean codepath, which was previously untested.

These tests would have caught the regression introduced by #20340 where
build_pooling() crashes with a ggml_mul_mat assertion due to mismatched
dimensions in the chunked GDN detection path.

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Co-authored-by: Domenico Crupi <redacted>