fix(jsonrpc): suppress error responses for notifications - #4146

Thanks for the contribution!

Please take a look at the inline comment.

Nit: the PR body lists utils/broadcast and core/trie2/trieutils under validation. Those packages are not touched, and are irrelevant to your changes.

Thanks for the update. One check is good, but the fix moved the whole dispatch into a nested if/else, and that brings its own problem:

• tuple is declared up front and stays nil on both error paths. The only thing that stops tuple[errorIndex] from panicking is the if res.Error != nil return above it. That link is implicit, and the next person to edit this function will not see it.
• The happy path now sits three levels deep.

Keep the gate in one place, but put it outside the dispatch instead of inside it. Something like:

1. Move the body of handleRequest from res := &response{...} to the end into a new method callMethod(ctx, req) (*response, http.Header). Keep its original early returns as they were on main. Drop the error return value, it was always nil past isSane.
2. Remove the res.ID == nil check from inside the dispatch.
3. Leave handleRequest as:

if err := req.isSane(); err != nil {
      s.logger.Trace("Request sanity check failed", zap.Error(err))
      return nil, http.Header{}, err
}

res, header := s.callMethod(ctx, req)
if req.ID == nil { // notification
      s.logger.Trace("Notification received, no response expected")
      return nil, header, nil
}
return res, header, nil