feat(core): implement CacheProvider trait

[JuliobaCR]

Description

Adds CacheProvider, the shared async contract (get/put/remove/clear) that cache backends implement, so the rest of the codebase can depend on "a cache" without caring which storage mechanism backs it. This is the foundational trait that the planned memory, disk, and Wasm-specific caches (#403, #404, #405) will implement.

How It Was Done

• crates/core/src/cache/provider.rs: CacheProvider trait, generic over the value type (Serialize / DeserializeOwned + Send / Sync); keys stay &str, consistent with the existing CacheStore.
• Methods return impl Future<..> + Send (RPITIT) rather than async fn. A plain async fn in a trait doesn't guarantee the returned future is Send, and backends that offload I/O via tokio::spawn / spawn_blocking need that guarantee — this also avoids the boxing overhead of #[async_trait].
• crates/core/src/error.rs: extends GratError with dedicated CacheMiss, CacheCapacityExceeded, CacheSerializationError, and CacheDeserializationError variants, so backends can surface typed errors instead of ad-hoc strings. The existing CacheError(String) variant is left as-is since CacheStore already depends on it.
• Adds a hand-rolled in-memory conformance test double (no new dependency) implementing CacheProvider, plus a trait-level test suite any future backend's own tests can mirror.

Issues Encountered (If Any)

• get's V bound needed an explicit Send (not just DeserializeOwned) for the returned future to actually be Send — the value type is part of the future's Output, so it's included in the auto-trait computation even though it's only read after the await point.
• A pre-existing cargo build --workspace --all-targets --all-features failure in grat-wasm (rlib/dylib mismatch for wasmparser/rustls/protobuf) reproduces identically on a clean main checkout, unrelated to this change.

Related Issue

Closes #402

How It Was Tested

cargo test -p grat-core --lib cache:: — 9/9 passing (7 new + 2 existing CacheStore tests): get-after-put, put-overwrites, miss returns Ok(None) (not an error), remove, remove-of-missing-key, clear, and over-capacity returns a typed error.

cargo fmt --all -- --check and cargo clippy -p grat-core --lib --tests -- -D warnings are clean for the new code (pre-existing clippy findings elsewhere in the crate are untouched by this PR).

Screenshots / Video (If Applicable)

N/A — trait-only change, no UI impact.

[coderabbitai]

Important

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[codeZe-us]

PR reviewed