fix(httputil): rename error body field platformCode → platform_code

errorBody() map key corrected from "platformCode" to "platform_code" to
match the OpenAPI spec snake_case convention. All HTTP error responses
produced by Error(w, err) now emit "platform_code" in the JSON body.
PR.md added to .gitignore.
Depends on xerrors v1.0.2 (go.mod bumped in follow-up commit after xerrors tag).

response.go

@@ -45,7 +45,7 @@ func errorBody(code, platformCode, message string, fields map[string]any) map[st
 		"message": message,
 	}
 	if platformCode != "" {
-		m["platformCode"] = platformCode
+		m["platform_code"] = platformCode
 	}
 	for k, v := range fields {
 		m[k] = v