Streamline response codes across API modules

[tk-o]

Stems from this feedback point:

• refactor(ensnode-sdk): api module #1353 (comment)

It seems there's an opportunity to share this pattern of ok vs error across each of our APIs, such that we don't have to repeat this implementation for each one.

This task requires creating a shared implementation for response codes to be used across all API modules (i.e. Indexing Status API, Registrar Actions API, Referrer Leaderboard API, etc.).

[tk-o]

Also, please update code docs to better describe what ok and error response codes mean for each relevant API module.

• https://github.com/namehash/ensnode/pull/1353/files#r2589894093

[tk-o]

When working on this task, please note the discussion about always returning 2xx HTTP status code, regardless of the responseCode field value.

@shrugs shared the following idea in this slack thread:

(...) we should only return 200 status codes from our APIs even if they include an error state, and then the HTTP status codes are reserved for HTTP level information, like arguments (400s) and whether the API had an internal error (500s). and any error State related to the API itself is encoded as a successful 200 response. then our SDK methods can safely throw when they see a non-200 error code, which is what I would expect if a fetch request failed.