fix: avoid unused async trait handlers

[lucarlig]

Summary

• replace synchronous async fn trait implementations with std::future::ready
• remove the need for Rust-version-specific Clippy allowances
• keep behavior unchanged across test fixtures and the secrets-detection plugin

This is the bottom PR beneath #107 and #109.

Verification

• cargo fmt --all -- --check
• Rust 1.96 and 1.98 workspace Clippy with warnings denied
• cargo nextest run --locked --workspace (223 passed, 3 skipped)
• workspace build, dependency policy, and shear checks