[Bug] AccountHandle::token_balance() returns 0 for freshly minted MockToken balances

[ekelemepraise-code]

Summary

After minting tokens to an account using MockToken::mint(), calling account.token_balance(&token) returns 0 instead of the minted amount. The balance is only visible through token.balance(&account.address()).

Steps to Reproduce

let env = MockEnv::builder()
    .with_account("alice", Stroops::xlm(1_000))
    .build();

let usdc = MockToken::new(&env, "USDC", 6);
let alice = env.account("alice");

usdc.mint(&alice.address(), 5_000_000);

// Bug: returns 0
let balance = alice.token_balance(&usdc);
assert_eq!(balance, 5_000_000); // FAILS

// Workaround: query directly on token
let balance2 = usdc.balance(&alice.address());
assert_eq!(balance2, 5_000_000); // passes

Root Cause (Hypothesis)

AccountHandle::token_balance may hold a stale reference or query the wrong storage key. Needs investigation in builders.rs.

Acceptance Criteria

• AccountHandle::token_balance(&token) returns live balance
• Regression test added
• Docs updated to clarify the difference between the two query paths