Title: test: add integration test for sell reverting when quantity exceeds holder balance

[chidii]

• Buys exactly 2 keys for a holder
• Attempts to sell 3 keys (exceeds balance)
• Asserts revert with ContractError::InsufficientBalance
• Asserts holder balance unchanged at 2
• Asserts creator supply unchanged

Closes #554

[Chucks1093]

❌ CI Failed — verify (Contracts CI)

The verify check is failing on this PR. This is a long-open PR — the most common issue for older PRs is that the branch is out of date with main.

Likely causes:

• Branch has diverged significantly from main — rebase needed
• Soroban SDK version mismatch (SDK has had breaking changes since this PR was opened)
• Compile errors from conflicts or stale dependencies

Steps to fix:

1. Rebase on latest main: git fetch origin && git rebase origin/main
2. Resolve any conflicts, then run cargo build
3. Update Cargo.toml if the Soroban SDK version in main has changed
4. Run cargo fmt --all and push