Remove async-trait from EncryptionService in favour of native async fn in traits (RPITIT)

[coderdan]

Context

EncryptionService (packages/cipherstash-proxy/src/proxy/mod.rs:152) is declared with #[async_trait::async_trait]. Native async fn in traits stabilised in Rust 1.75 (Dec 2023); the proxy builds on a much newer toolchain (currently 1.92) with no pinned MSRV, so the async-trait crate is no longer required by the language.

This is a code-hygiene follow-up surfaced while adding the BUG-300 regression test (#400 / PR #395) — not a blocker for that PR.

Why it isn't a trivial deletion

The encrypt/decrypt futures are driven inside a per-connection handler spawned on the multithreaded tokio runtime (main.rs tracker.spawn(...) → pg::handler().await → backend.rs:488 self.context.decrypt(...).await). For that spawned task to be Send, the futures held across .await must be Send.

#[async_trait] desugars each method to Pin<Box<dyn Future + Send + '_>>, so it injects the Send bound for free. **Bare native async fn in a trait does not guarantee a Send future** through a generic S: EncryptionService, so a naive removal will fail to compile at the spawn` boundary.

Proposed approach

Convert to RPITIT with an explicit Send bound on the trait methods:

pub trait EncryptionService: Send + Sync {
    fn encrypt(&self, /* … */)
        -> impl std::future::Future<Output = Result<Vec<Option<EqlCiphertext>>, Error>> + Send;

    fn decrypt(&self, /* … */)
        -> impl std::future::Future<Output = Result<Vec<Option<Plaintext>>, Error>> + Send;
}

Impls can keep async fn as long as their concrete future is Send. (trait-variant's #[trait_variant::make(Send)] is an alternative that reduces boilerplate.)

This is viable because there is no dyn EncryptionService anywhere — all usage is generic/monomorphised — so RPITIT's lack of dyn-compatibility is not a blocker. (There is org precedent for dropping async-trait in another crate.)

Sites to update

• Trait definition: packages/cipherstash-proxy/src/proxy/mod.rs:151-152
• Production impl: packages/cipherstash-proxy/src/proxy/zerokms/zerokms.rs:150 (ZeroKms)
• Test impls: packages/cipherstash-proxy/src/postgresql/context/mod.rs:1073 and packages/cipherstash-proxy/src/postgresql/backend.rs:745 (TestService)
• Drop the dependency: async-trait = "0.1" in packages/cipherstash-proxy/Cargo.toml (only if no other #[async_trait] users remain)

Acceptance criteria

• EncryptionService no longer uses #[async_trait]; spawned connection handling still compiles (Send preserved).
• async-trait removed from Cargo.toml if it has no remaining users.
• cargo fmt --check, cargo clippy --all-features --tests -- -D warnings, and the test suite all pass.