fix: unused import 'vec' in dex_router.rs causes Clippy -D warnings CI failure

[Pvsaint]

Problem

dex_router.rs line 1 imports vec from soroban_sdk but never uses it:

use soroban_sdk::{contract, contractimpl, Address, Env, Vec, vec};
//                                                           ^^^  unused

CI runs cargo clippy --all-targets --all-features -- -D warnings, so this warning is promoted to an error and blocks the pipeline.

Acceptance Criteria

• vec is removed from the import in dex_router.rs
• cargo clippy --all-targets --all-features -- -D warnings produces zero warnings/errors for this file
• No existing functionality is broken

Branch & Commit Examples

branch: fix/clippy-warnings-cleanup
commit: fix(dex-router): remove unused vec import