Write UT for public methods if it is possible

### TL;DR

Write UT for public methods instead of private methods if it is possible

https://github.com/abcxyz/jvs/blob/23cdda185a8fc277ba798bfaf3e34543458f1934/pkg/jvscrypto/cert_action_api_test.go#L306 and
https://github.com/abcxyz/jvs/blob/23cdda185a8fc277ba798bfaf3e34543458f1934/pkg/jvscrypto/key_hosting_test.go#L124

### Detailed design

_No response_

### Alternatives considered

_No response_

### Additional information

_No response_

Look at the two linked test files: pkg/jvscrypto/cert_action_api_test.go and pkg/jvscrypto/key_hosting_test.go. Identify the public methods being tested indirectly via private methods. Refactor the tests to directly test the public methods instead. Run the existing tests to ensure they still pass after the change.

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