Validate cookie policy link in banner description

Translations are no longer part of the version snapshot, so the
banner_description text must be validated at write time to ensure
the {{cookie_policy_link}} placeholder is present. Without it
the cookie policy URL silently disappears from the rendered banner.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/validator/validator_string_test.go

@@ -67,6 +67,31 @@ func TestMaxLen(t *testing.T) {
 	}
 }
 
+func TestContainsSubstring(t *testing.T) {
+	tests := []struct {
+		name      string
+		value     any
+		substr    string
+		wantError bool
+	}{
+		{"contains substring", "hello {{cookie_policy_link}} world", "{{cookie_policy_link}}", false},
+		{"missing substring", "hello world", "{{cookie_policy_link}}", true},
+		{"exact match", "{{cookie_policy_link}}", "{{cookie_policy_link}}", false},
+		{"empty string", "", "{{cookie_policy_link}}", true},
+		{"nil pointer", (*string)(nil), "{{cookie_policy_link}}", false},
+		{"non-string", 123, "foo", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ContainsSubstring(tt.substr)(tt.value)
+			if (err != nil) != tt.wantError {
+				t.Errorf("ContainsSubstring() error = %v, wantError %v", err, tt.wantError)
+			}
+		})
+	}
+}
+
 func TestOneOf(t *testing.T) {
 	tests := []struct {
 		name      string