Add stagger delay to fix frontdoor login

[kckrithika]

Summary

Resolved a race condition in the frontdoor login flow by implementing a locking mechanism to ensure the singleaccess endpoint is accessed sequentially rather than in parallel.

The Problem
The singleaccess endpoint generates a one-time-use frontdoor URL that is invalidated the moment the endpoint receives a new request.

Bug
Multiple "frontdoor login" attempts were hitting the endpoint parallelly. A race condition occurred where a second request would invalidate the URL generated by the first before the authentication flow could complete, leading to intermittent login failures.

The Fix
The test script now calls the endpoint sequentially. The main scripts now have a Lock to synchronize access to the singleaccess endpoint. The lock ensures that only one request can hit the endpoint at a time.

[kckrithika]

@shashacks

Here's the PR to fix Issue 6. Could you please check if this fix works for you?

Please check out to frontdoor-access-fix branch and follow the same instructions in README to run the test login.

[shashacks]

@kckrithika
Confirmed the fix works on my end —
--num_instances 2 returns 2/2 and --num_instances 4 returns 4/4 with no failures. Thanks!