Add invalid balance lookup test coverage (#32)

[haanarrh]

Summary

• Added comprehensive test coverage for invalid balance and supply lookups in creator-keys.
• Verified that read-only methods like get_key_balance, get_total_key_supply, and get_creator_holder_count return 0 for unregistered creators, avoiding panics.
• Confirmed that get_creator_supply correctly returns Err(ContractError::NotRegistered) for missing creators.
• Added edge case tests for get_key_balance where the creator is the holder and where the contract address itself is the holder.
• Validated that get_creator_details returns a structured view with is_registered: false and default field values for unregistered creators.

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace (New tests in invalid_balance_lookups.rs verify all edge cases)

Checklist

• Linked issue or backlog item: Closes Add invalid balance lookup test coverage #32
• Contract behavior and invariants are described clearly
• Docs updated if contract interfaces or workflows changed (No interface changes, only test additions)
• Scope stays limited to one contract concern

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[haanarrh]

please kindly review