fix: add conditions for minimum timedelta in scheduling

[abhigyanghosh30]

Done

• Should not be possible to schedule exams within 30 mins

QA

• Check out this feature branch
• Run the site using the command ./run serve or dotrun
• View the site locally in your web browser at: http://0.0.0.0:8001/
  • Be sure to test on mobile, tablet and desktop screen sizes
• Purchase an exam
• Try to schedule it within 30 mins
• Should get an error

Issue / Card

Fixes #WD-7947

Screenshots

[If relevant, please include a screenshot.]

Help

QA steps - Commit guidelines

[webteam-app]

Demo starting at https://ubuntu-com-13408.demos.haus

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (722fea2) 74.41% compared to head (db59858) 74.41%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13408   +/-   ##
=======================================
  Coverage   74.41%   74.41%           
=======================================
  Files         107      107           
  Lines        2838     2838           
  Branches      946      946           
=======================================
  Hits         2112     2112           
  Misses        702      702           
  Partials       24       24