forcing /checkout to fail via validate_inventory: "true"

[ndmanvar]

flask's /checkout does not error out unless you now pass it validate_inventory: "true" in the body

This change was made a few days ago via sentry-demos/empower#573

for some reason /checkout transaction was not being sent in the success case 🤷
regardless, adding validate_inventory: "true" gets us back to a working state in terms of error and transaction (see below)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (f7ce3a9) to head (f584096).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
.../example/vu/android/empowerplant/MainFragment.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #152   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         15      15           
  Lines        779     780    +1     
  Branches      43      43           
=====================================
- Misses       779     780    +1     

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