feat(config): default prompt_cache_all to true (#9951)

Upstream llama.cpp defaults `cache_prompt = true` (common/common.h),
but `parse_options` in the grpc-server backend unconditionally forwards
the proto `PromptCacheAll` field, so any model that didn't set
`prompt_cache_all: true` in its YAML was getting `cache_prompt=false` —
silently overriding llama.cpp's own default. With `kv_unified` and
`cache_idle_slots` already on by default, this was the last piece
preventing the per-request prompt cache from being usable out of the
box.

Make `PromptCacheAll` tristate (`*bool`), default it to `true` in
`SetDefaults`, and dereference at the proto boundary. Users can still
opt out with an explicit `prompt_cache_all: false`. Same pattern as
`MMap`, `MMlock`, `Reranking`, etc.

Co-authored-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>