Add Cargo Clippy command to Makefile and fix warnings

[Code-Paragon]

Problem

The project relies on make fmt and make test but lacks a dedicated linting step to catch Rust idiomatic errors, unused imports, or potential memory/performance issues.

Expected behavior

A lint target is added to the Makefile utilizing cargo clippy, and any existing warnings in the codebase are resolved.

Files to update

• Makefile
• src/lib.rs
• src/vault.rs
• src/strategy.rs
• src/test.rs

Project relevance

Integrating Clippy ensures the Soroban smart contracts remain optimized, secure, and idiomatic as the open-source contributor base scales.

Acceptance criteria

• Feature works as described
• Tests are added or updated where needed
• Documentation is updated where needed
• Makefile includes lint: cargo clippy --all-targets --all-features -- -D warnings
• No TypeScript, lint, test, or build errors