test(creator-keys): add coverage for creator fee recipient update val…

[funds0033-cmyk]

Adds focused unit tests for the update_creator_fee_recipient functionality to verify successful updates, authorization checks, and zero-address validation.

What was added
Added creator-keys/tests/update_creator_fee_recipient.rs.
Added test coverage for:
Valid fee recipient updates are persisted correctly.
Zero-address recipients are rejected with ContractError::ZeroAddress.
Unauthorized callers cannot update the fee recipient and no state changes occur.
A second valid update replaces the previous recipient.
The previous recipient is no longer stored after an update.
Notes
The issue description references an invalid_recipient error, but the contract currently returns ContractError::ZeroAddress. The tests were written against the contract's actual behavior.
This PR only adds test coverage and does not modify contract logic.
Validation
Local cargo test could not be executed because the local environment is missing the MSVC link.exe toolchain.
The tests are intended to be validated by the project's GitHub CI.
Closes #681

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