Plan and implement unit tests

[Siegrift]

The API package has reasonable unit test coverage and does not need additional tests.

The pusher package would benefit from testing the logic for making requests, that is tests for these files:

• https://github.com/api3dao/signed-api/tree/83b6b1624399289d4ae71864b6e2071372272fec/packages/data-pusher/src/api-requests