feat: include platformCode in error responses (v0.10.0)

- Error(w, err) now extracts PlatformCode() from *xerrors.Err and
  includes "platformCode" in the JSON body when set; omitted otherwise
- errorBody updated to accept platformCode as an explicit parameter
- Upgraded code.nochebuena.dev/go/xerrors dependency to v0.10.0
- Added two new tests: platformCode included when set, omitted when absent

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

httputil_test.go

@@ -240,6 +240,28 @@ func TestError_NilError(t *testing.T) {
 	}
 }
 
+func TestError_PlatformCode_IncludedWhenSet(t *testing.T) {
+	rec := httptest.NewRecorder()
+	Error(rec, xerrors.New(xerrors.ErrNotFound, "employee not found").
+		WithPlatformCode("EMPLOYEE_NOT_FOUND"))
+	if rec.Code != http.StatusNotFound {
+		t.Errorf("want 404, got %d", rec.Code)
+	}
+	m := decodeMap(t, rec)
+	if m["platformCode"] != "EMPLOYEE_NOT_FOUND" {
+		t.Errorf("platformCode: want EMPLOYEE_NOT_FOUND, got %v", m["platformCode"])
+	}
+}
+
+func TestError_PlatformCode_OmittedWhenNotSet(t *testing.T) {
+	rec := httptest.NewRecorder()
+	Error(rec, xerrors.New(xerrors.ErrInternal, "unexpected error"))
+	m := decodeMap(t, rec)
+	if _, ok := m["platformCode"]; ok {
+		t.Errorf("platformCode should be absent for errors without a platform code, got %v", m["platformCode"])
+	}
+}
+
 func TestErrorCodeToStatus_AllCodes(t *testing.T) {
 	cases := []struct {
 		code   xerrors.Code