clippy 1.99.0 `double_must_use` fails on async_trait-generated `Blob` trait

[fnando]

Rust 1.99.0 ships a new clippy::double_must_use lint that fails make check (and therefore CI, e.g. #2783). The failure is in cmd/crates/stellar-ledger/src/signer.rs:

error: this function has a `#[must_use]` attribute with no message, but returns a type already considered as `#[must_use]`
 --> cmd/crates/stellar-ledger/src/signer.rs:1:28
  = note: the return type is pinned boxed `std::future::Future` trait object
  = note: `-D clippy::double-must-use` implied by `-D clippy::all`
  = note: this error originates in the attribute macro `async_trait::async_trait`

Cause

The #[async_trait::async_trait] macro on the Blob trait expands each async fn to a method that both (a) carries an explicit #[must_use] and (b) returns Pin<Box<dyn Future + Send>>, which clippy 1.99.0 now treats as already #[must_use]. The two together trip double_must_use. This is macro-generated code we do not control, so the lint is effectively a false positive for async-trait (already on the latest 0.1.89).

Options

1. Suppress narrowly: add #[allow(clippy::double_must_use)] to the Blob trait. Smallest change, but hides the lint for that trait.
2. Drop async_trait for native async-fn-in-trait (stable since 1.75). Blob is never used as dyn Blob (only as a bound and for method calls), so it can become:

use std::future::Future;

pub trait Blob {
    type Key: Send;
    type Error;
    fn get_public_key(
        &self,
        key: &Self::Key,
    ) -> impl Future<Output = Result<stellar_strkey::ed25519::PublicKey, Self::Error>> + Send;
    fn sign_blob(
        &self,
        key: &Self::Key,
        blob: &[u8],
    ) -> impl Future<Output = Result<Vec<u8>, Self::Error>> + Send;
}

with the #[async_trait::async_trait] attribute removed from the impl Blob for LedgerSigner<T> block in cmd/crates/stellar-ledger/src/lib.rs (the async fn bodies stay as-is). The explicit + Send return bound preserves the current Send semantics. This removes the lint-triggering macro entirely and is the modern idiom.

Note: the same crate still uses #[async_trait] in emulator_test_support/http_transport.rs for a different trait, which may hit the same lint once those feature-gated targets compile.

Impact

make check / CI is red for anyone on Rust 1.99.0 until this is addressed.