* llama: honor PARTIAL_ONLY in the plain KV cache

A non-SWA llama_kv_cache rolls back with seq_rm, so it holds no
partial state, yet state_write ignored the flag and serialized every
cell. A draft context made of a plain KV cache (MTP head, dense -md
draft) therefore copied its whole KV into each server checkpoint,
growing it by the draft KV size per token.

The cache now writes and reads nothing for PARTIAL_ONLY unless it is
SWA, and the iSWA wrapper no longer gates its base cache since it
filters itself.

* Update src/llama-kv-cache.cpp

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

* llama: delegate PARTIAL_ONLY to the caches of every memory wrapper

The hybrid, hybrid-idx and dsa-iswa wrappers skipped their attention
caches on PARTIAL_ONLY, which also dropped an SWA window that is
partial state. Each cache now applies the flag itself, so the
wrappers only delegate. The indexer is marked stale only when a
restore rewrites cells.

test-llama-context gains a case checking that the partial state is
not larger than the full state and, without SWA, does not grow with
the tokens.

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>