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>

response.go

@@ -22,28 +22,31 @@ func NoContent(w http.ResponseWriter) {
 }
 
 // Error maps err to the appropriate HTTP status code and writes a JSON error body.
-// Understands *xerrors.Err — extracts Code and Message; fields are included if present.
+// Understands *xerrors.Err — extracts Code, PlatformCode, and Message.
 // Falls back to 500 for unknown errors.
 func Error(w http.ResponseWriter, err error) {
 	if err == nil {
-		JSON(w, http.StatusInternalServerError, errorBody("INTERNAL", "internal server error", nil))
+		JSON(w, http.StatusInternalServerError, errorBody("INTERNAL", "", "internal server error", nil))
 		return
 	}
 	var xe *xerrors.Err
 	if errors.As(err, &xe) {
 		status := errorCodeToStatus(xe.Code())
-		body := errorBody(string(xe.Code()), xe.Message(), xe.Fields())
+		body := errorBody(string(xe.Code()), xe.PlatformCode(), xe.Message(), xe.Fields())
 		JSON(w, status, body)
 		return
 	}
-	JSON(w, http.StatusInternalServerError, errorBody("INTERNAL", "internal server error", nil))
+	JSON(w, http.StatusInternalServerError, errorBody("INTERNAL", "", "internal server error", nil))
 }
 
-func errorBody(code, message string, fields map[string]any) map[string]any {
+func errorBody(code, platformCode, message string, fields map[string]any) map[string]any {
 	m := map[string]any{
 		"code":    code,
 		"message": message,
 	}
+	if platformCode != "" {
+		m["platformCode"] = platformCode
+	}
 	for k, v := range fields {
 		m[k] = v
 	}