fix(contracts): satisfy clippy in usdc_lending_pool

Name the elided lifetime on usdc_client's return type and prefix the
unused test binding, so `cargo clippy --all-targets -D warnings` passes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: thisisouvik

contracts/usdc_lending_pool/src/lib.rs

@@ -368,7 +368,7 @@ impl UsdcLendingPool {
     }
 
     /// Build a typed SEP-41 token client for the USDC contract.
-    fn usdc_client(env: &Env) -> token::TokenClient {
+    fn usdc_client(env: &Env) -> token::TokenClient<'_> {
         let addr: Address = env
             .storage()
             .instance()

contracts/usdc_lending_pool/src/test.rs

@@ -138,7 +138,7 @@ fn test_deposit_rejects_negative_amount() {
 
 #[test]
 fn test_deposit_accumulates_on_second_deposit() {
-    let (env, pool_id, _admin, user, usdc_addr) = setup();
+    let (env, pool_id, _admin, user, _usdc_addr) = setup();
     let client = UsdcLendingPoolClient::new(&env, &pool_id);
 
     // First deposit