chore: fix regex as per codeQL rec

Author: GautamSharda

handwritten/pubsub/src/pubsub.ts

@@ -806,7 +806,7 @@ export class PubSub {
 
     // Parse the URL into a hostname and port, if possible.
     const leadingProtocol = new RegExp('^https?://');
-    const trailingSlashes = new RegExp('/*$');
+    const trailingSlashes = new RegExp('(?<!/)/+$');
     const baseUrlParts = apiEndpoint!
       .replace(leadingProtocol, '')
       .replace(trailingSlashes, '')