chore: Refactor test setup for Gin framework

- Removed redundant `gin.SetMode(gin.TestMode)` calls from individual test files.
- Introduced a centralized `TestMain` function in `testmain_test.go` to set the Gin mode for all tests.
- Ensured consistent test environment setup across various handler test files.

backend/internal/api/handlers/testmain_test.go

@@ -0,0 +1,13 @@
+package handlers
+
+import (
+	"os"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestMain(m *testing.M) {
+	gin.SetMode(gin.TestMode)
+	os.Exit(m.Run())
+}