llama : refactor kv cache guard (#12695)
* llama : refactor kv cache guard
ggml-ci
* cont : fix comment [no ci]
* llama : fix kv_cache restore logic
ggml-ci
* context : simplify kv cache updates
ggml-ci
* cont : better name [no ci]
* llama : fix llama_decode return code when could not find KV slot
ggml-ci
* context : change log err -> warn [no ci]
* kv-cache : add comment + warning