♻️ Use json.NewEncoder for HTTP JSON responses

Replace manual fmt.Fprintf JSON string formatting with proper
json.NewEncoder for health and readiness handlers. This prevents
potential issues with unescaped characters in error messages and
ensures consistent JSON output.

Co-Authored-By: OpenCode <noreply@opencode.ai>

Author: 0x46616c6b

prometheus.go

@@ -2,7 +2,7 @@ package main
 
 import (
 	"context"
-	"fmt"
+	"encoding/json"
 	"net/http"
 	"time"
 
@@ -110,7 +110,7 @@ func StartMetricsServer(ctx context.Context, listenAddr string, userliClient Use
 func healthHandler(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusOK)
-	fmt.Fprintf(w, `{"status":"ok"}`)
+	_ = json.NewEncoder(w).Encode(map[string]string{"status": "ok"})
 }
 
 // readyHandler handles readiness probe requests (checks Userli API connectivity)
@@ -134,17 +134,17 @@ func readyHandler(userliClient UserliService) http.HandlerFunc {
 			if err != nil {
 				healthCheckStatus.Set(0)
 				w.WriteHeader(http.StatusServiceUnavailable)
-				fmt.Fprintf(w, `{"status":"unavailable","error":"%s"}`, err.Error())
+				_ = json.NewEncoder(w).Encode(map[string]string{"status": "unavailable", "error": err.Error()})
 				logger.Warn("Readiness check failed", zap.Error(err))
 				return
 			}
 			healthCheckStatus.Set(1)
 			w.WriteHeader(http.StatusOK)
-			fmt.Fprintf(w, `{"status":"ready"}`)
+			_ = json.NewEncoder(w).Encode(map[string]string{"status": "ready"})
 		case <-ctx.Done():
 			healthCheckStatus.Set(0)
 			w.WriteHeader(http.StatusServiceUnavailable)
-			fmt.Fprintf(w, `{"status":"unavailable","error":"timeout"}`)
+			_ = json.NewEncoder(w).Encode(map[string]string{"status": "unavailable", "error": "timeout"})
 			logger.Warn("Readiness check timeout")
 		}
 	}

prometheus_test.go

@@ -25,7 +25,7 @@ func (s *PrometheusTestSuite) TestHealthHandler() {
 
 		s.Equal(http.StatusOK, w.Code)
 		s.Equal("application/json", w.Header().Get("Content-Type"))
-		s.Equal(`{"status":"ok"}`, w.Body.String())
+		s.Equal("{\"status\":\"ok\"}\n", w.Body.String())
 	})
 }
 
@@ -42,7 +42,7 @@ func (s *PrometheusTestSuite) TestReadyHandler() {
 
 		s.Equal(http.StatusOK, w.Code)
 		s.Equal("application/json", w.Header().Get("Content-Type"))
-		s.Equal(`{"status":"ready"}`, w.Body.String())
+		s.Equal("{\"status\":\"ready\"}\n", w.Body.String())
 
 		mockClient.AssertExpectations(s.T())
 	})
@@ -59,7 +59,7 @@ func (s *PrometheusTestSuite) TestReadyHandler() {
 
 		s.Equal(http.StatusServiceUnavailable, w.Code)
 		s.Equal("application/json", w.Header().Get("Content-Type"))
-		s.Equal(`{"status":"unavailable","error":"connection refused"}`, w.Body.String())
+		s.Equal("{\"error\":\"connection refused\",\"status\":\"unavailable\"}\n", w.Body.String())
 
 		mockClient.AssertExpectations(s.T())
 	})
@@ -80,7 +80,7 @@ func (s *PrometheusTestSuite) TestReadyHandler() {
 
 		s.Equal(http.StatusServiceUnavailable, w.Code)
 		s.Equal("application/json", w.Header().Get("Content-Type"))
-		s.Equal(`{"status":"unavailable","error":"timeout"}`, w.Body.String())
+		s.Equal("{\"error\":\"timeout\",\"status\":\"unavailable\"}\n", w.Body.String())
 	})
 }