test: Add regression test for key balance read returning zero

[AbuJulaybeeb]

Added a new regression test test_key_balance_read_returns_zero_for_address_with_no_buys to creator-keys/tests/key_balance.rs. This explicitly tests querying the key balance using a distinct, uninitialized wallet address that has not interacted with the contract.

Why it was done

To resolve #369 by guaranteeing that a valid wallet address that has never bought keys for a creator returns a zero balance natively. This confirms the read handles uninitialized holders gracefully rather than producing unexpected panics, errors, or null states.

How it was verified

The regression test initializes the environment, registers a creator, simulates an active buyer interacting with the contract, and queries the inactive_wallet. It successfully asserts that client.get_key_balance(&creator, &inactive_wallet) evaluates to 0 without throwing any errors.
Closes #369

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