refactor: Relocate JavaScript source and Node.js test files to dedicated directories and extract OpenAI stream runtime tool call finalization logic.

internal/js/chat-stream/error_shape.js

@@ -0,0 +1,36 @@
+'use strict';
+
+function writeOpenAIError(res, status, message) {
+  res.statusCode = status;
+  res.setHeader('Content-Type', 'application/json');
+  res.end(
+    JSON.stringify({
+      error: {
+        message,
+        type: openAIErrorType(status),
+      },
+    }),
+  );
+}
+
+function openAIErrorType(status) {
+  switch (status) {
+    case 400:
+      return 'invalid_request_error';
+    case 401:
+      return 'authentication_error';
+    case 403:
+      return 'permission_error';
+    case 429:
+      return 'rate_limit_error';
+    case 503:
+      return 'service_unavailable_error';
+    default:
+      return status >= 500 ? 'api_error' : 'invalid_request_error';
+  }
+}
+
+module.exports = {
+  writeOpenAIError,
+  openAIErrorType,
+};