Add focused zero-balance key read tests

[Abidoyesimze]

Description

Adds focused tests for zero-balance key read behavior in creator-keys.
Added tests
test_key_balance_zero_for_unregistered_creator_even_when_other_balances_exist
Confirms get_key_balance returns 0 for an unregistered creator, even when other creator/buyer balances exist.
test_key_balance_zero_for_registered_creator_and_unseen_wallet
Confirms get_key_balance returns 0 for a registered creator when the queried wallet has never purchased.

Why

We want explicit coverage of zero-balance read paths so read behavior is clear and stable for indexers/clients:
missing creator + read returns 0
missing creator-wallet balance entry + read returns 0

Scope

Test-only change.
No contract logic changes.
Tests remain small and focused on read behavior.

Validation

Ran local CI-equivalent checks successfully:
make fmt-check
make clippy
make test
All checks passed.

closes #40

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