test: improve pkg package coverage

- Add HTTP status error functions tests (internal/pkg/errors)
- Add ReadRequestBodyWithPrealloc tests (internal/pkg/httputil)
- Add HTTPStatusToGoogleStatus tests (internal/pkg/googleapi)

Coverage improvements:
- pkg/errors: 77.6%
- pkg/httputil: 91.7%
- pkg/googleapi: 79.5%

internal/pkg/googleapi/status_test.go

@@ -0,0 +1,40 @@
+package googleapi
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestHTTPStatusToGoogleStatus(t *testing.T) {
+	tests := []struct {
+		name   string
+		status int
+		want   string
+	}{
+		{"bad_request_400", http.StatusBadRequest, "INVALID_ARGUMENT"},
+		{"unauthorized_401", http.StatusUnauthorized, "UNAUTHENTICATED"},
+		{"forbidden_403", http.StatusForbidden, "PERMISSION_DENIED"},
+		{"not_found_404", http.StatusNotFound, "NOT_FOUND"},
+		{"too_many_requests_429", http.StatusTooManyRequests, "RESOURCE_EXHAUSTED"},
+		{"internal_server_500", http.StatusInternalServerError, "INTERNAL"},
+		{"bad_gateway_502", http.StatusBadGateway, "INTERNAL"},
+		{"service_unavailable_503", http.StatusServiceUnavailable, "INTERNAL"},
+		{"ok_200", http.StatusOK, "UNKNOWN"},
+		{"created_201", http.StatusCreated, "UNKNOWN"},
+		{"accepted_202", http.StatusAccepted, "UNKNOWN"},
+		{"no_content_204", http.StatusNoContent, "UNKNOWN"},
+		{"bad_request_boundary", 400, "INVALID_ARGUMENT"},
+		{"server_error_boundary", 500, "INTERNAL"},
+		{"custom_4xx", 418, "UNKNOWN"},
+		{"custom_5xx", 599, "INTERNAL"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := HTTPStatusToGoogleStatus(tt.status)
+			require.Equal(t, tt.want, got)
+		})
+	}
+}