chore(backend): return a ready future from the composite exchange test mock

[AntonioVentilii]

Motivation

Rust 1.98 adds clippy::unused_async_trait_impl, which fires on an async fn in a trait impl whose body contains no .await. The backend lint job runs with -D warnings, so it becomes a hard error:

error: unused `async` for async trait impl function with no `.await` statements
  --> src/backend/src/exchange/composite.rs:109:9
error: could not compile `backend` (lib test) due to 1 previous error

This is the only occurrence in the repo. PanickingPrimaryProvider::fetch_prices right below it is exempt because its body diverges on panic!.

It currently blocks the toolchain bump in #13776, which fails on nothing else. Landing it separately keeps that PR a pure dependabot bump.

Changes

MockPrimaryProvider::fetch_prices is a plain fn returning impl Future and wraps its value in std::future::ready, which is the form clippy suggests. Implementing a trait's async fn this way is allowed, and it compiles on 1.97.1 as well, so this does not depend on the bump landing first.

Tests

• cargo test -p backend --lib exchange::composite: 8 passed, 0 failed.
• ./scripts/lint.rust.sh: clean.

Both on the pinned 1.97.1. The 1.98 lint itself is verified by CI on #13776 once this is merged and that branch rebases.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 1cffb63.

Security Overview

• 🔎 Scanned files: 1 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/00881019-f162-4503-9567-953b0be667d7?scanId=d6699cd1-e8cb-49d9-9565-468d02403c35&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Refactor	► src/backend/src/exchange/composite.rs     Modify test utilities imports to include future::Future and ready, and rc::Rc; adjust fetch_prices: from async fn to fn returning impl Future, using ready(...) to wrap result

[Copilot]

🟢 Approval recommended

The change is narrowly scoped to test-only code, matches the trait’s desugared async signature, and should resolve the Rust 1.98 clippy failure without affecting runtime behavior.

Pull request overview

Adjusts the exchange::composite unit-test mock to avoid Rust 1.98’s new clippy::unused_async_trait_impl lint (which becomes a hard error under -D warnings), keeping the backend lint gate green during the toolchain bump.

Changes:

• Updates MockPrimaryProvider::fetch_prices to return a ready future (std::future::ready(...)) instead of using an async fn body with no .await.
• Updates test-module imports to include std::future::{ready, Future}.

File summaries

File	Description
src/backend/src/exchange/composite.rs	Updates the test mock provider implementation to return an immediately-ready future and avoid the new clippy lint.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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