feat(proxy): add upstream error and rate-limit tracking support

src/server/modules/proxy/clients/upstream-error.ts

@@ -0,0 +1,23 @@
+export interface UpstreamErrorHeaders {
+  retryAfter?: string;
+}
+
+export class UpstreamRequestError extends Error {
+  readonly status?: number;
+  readonly headers?: UpstreamErrorHeaders;
+  readonly body?: string;
+
+  constructor(params: {
+    message: string;
+    status?: number;
+    headers?: UpstreamErrorHeaders;
+    body?: string;
+  }) {
+    super(params.message);
+    this.name = 'UpstreamRequestError';
+    this.status = params.status;
+    this.headers = params.headers;
+    this.body = params.body;
+  }
+}
+