llama : clamp kpool re-pool bound to existing pools (#29805)

* tests : simplify function signature

* llama : clamp kpool re-pool bound to existing pools

The n_tokens/kpool + n_seqs_unq bound on n_new_g overshoots when a batch
fills the whole cache: n_ctx tokens complete exactly n_ctx/kpool pools, so
the +1 pads new_pool_idxs/new_pool_rep one entry past n_pool_real. Graph
reserve only covers n_pool_real entries, so the first full-context decode
builds bigger tensors than reserved and ggml-alloc demands a graph
reallocation (abort under GGML_SCHED_DEBUG_REALLOC=1).

Clamp the bound to n_pool_real: a ubatch can never mark more pools than
the cache holds, and reserve's n_pool_max already covers that.

Assisted-by: pi:llama.cpp/MiMo-V2.6-Flash-MOPD

* cont : cap to n_pool_max