fix(jsonrpc): report JSON contents even on incomplete/malformed responses

- Sets Content-Type: application/json before switching/parsing the request body.
- Removes redundant branch-specific assignments.
- Adds a regression test for malformed single-object input.

Author: renatomaia

internal/jsonrpc/batchcalls_test.go

@@ -157,6 +157,15 @@ func TestJSONRPCMalformedBatchReturnsParseErrorObject(t *testing.T) {
 	requireRPCError(t, decodeRPCResponse(t, rr.Body.Bytes()), nil, JSONRPC_PARSE_ERROR)
 }
 
+func TestJSONRPCMalformedObjectReturnsJSONContentType(t *testing.T) {
+	s := newBatchTestService()
+	rr := serveRPC(t, s, []byte(`{"jsonrpc":"2.0"`))
+
+	require.Equal(t, http.StatusOK, rr.Code)
+	require.Equal(t, "application/json", rr.Header().Get("Content-Type"))
+	requireRPCError(t, decodeRPCResponse(t, rr.Body.Bytes()), nil, JSONRPC_PARSE_ERROR)
+}
+
 func TestJSONRPCBatchMalformedElementDoesNotPoisonValidSiblings(t *testing.T) {
 	s := newBatchTestService()
 	body := []byte(`[

internal/jsonrpc/jsonrpc.go

@@ -331,6 +331,7 @@ func (s *Service) handleRPC(w http.ResponseWriter, r *http.Request) {
 	}
 
 	budgetResp := newBudgetWriter(w, MAX_RESPONSE_SIZE)
+	w.Header().Set("Content-Type", "application/json")
 
 	switch body[0] {
 	case '{':
@@ -339,12 +340,10 @@ func (s *Service) handleRPC(w http.ResponseWriter, r *http.Request) {
 			s.writeRPCError(w, nil, JSONRPC_PARSE_ERROR, "invalid request")
 			return
 		}
-		w.Header().Set("Content-Type", "application/json")
 		s.Logger.Info("Dispatching RPC request", "method", truncatedMethod(req.Method))
 		s.dispatchOneRequest(w, r, req, budgetResp)
 
 	case '[':
-		w.Header().Set("Content-Type", "application/json")
 		var reqSeq []json.RawMessage
 		if err := json.Unmarshal(body, &reqSeq); err != nil {
 			s.writeRPCError(w, nil, JSONRPC_PARSE_ERROR, "invalid request batch")
@@ -401,7 +400,6 @@ func (s *Service) handleRPC(w http.ResponseWriter, r *http.Request) {
 		s.writeByte(w, ']')
 
 	default:
-		w.Header().Set("Content-Type", "application/json")
 		if json.Valid(body) {
 			s.writeRPCError(w, nil, JSONRPC_INVALID_REQUEST, "invalid request")
 		} else {