hproxy: use JSON-RPC responses

### Confirmation

- [x] I have checked for similar issues.

### Problem

Hproxy should write responses in JSON-RPC format, rather than basic HTTP responses. This will allow for clearer errors and information being relayed to the client, as HTTP Status codes are currently 'abused' or over-generalized to fit the use-case.

### Suggested solution

Replace the current code to respond to requests with JSON-RPC responses and errors.

### Additional Information

_No response_

Start by locating hproxy's current request-response handling and reviewing how it currently maps outcomes to HTTP responses. Then identify the project's expected JSON-RPC response and error conventions; the work is done when hproxy consistently returns JSON-RPC responses instead of the current basic HTTP responses.

Written by the indexing model from the issue text.