Refactor: Call processDeliveryItem only on successful checkout

[sentry]

This PR addresses the MainFragment$BackendAPIException: Failed to init delivery workflow by modifying the Android client's checkout flow.

Previously, processDeliveryItem was called unconditionally after any HTTP response from the /checkout endpoint, including failures (e.g., 500s from the backend) or network exceptions. This led to the client throwing its own BackendAPIException even when the primary issue was a backend problem.

This change ensures that processDeliveryItem is only invoked when the HTTP checkout request returns a successful (2xx) status code. For non-successful responses or network failures, the checkout transaction is simply marked as INTERNAL_ERROR without attempting to initiate the delivery workflow.

Note: The underlying root cause of the 500 errors from the Flask backend (an UnboundLocalError due to quantities being used before assignment) is not addressed in this client-side PR. This change improves the client's resilience and error handling for such backend failures.

Fixes ANDROID-K7

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
Android	com.example.vu.android	24.12.26 (241226)	release
Android	com.example.vu.android	24.12.26 (241226)	debug
Android	com.example.vu.android	24.12.26 (241226)	release

⚙️ android Build Distribution Settings

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (410e6f1) to head (6a847b9).

Files with missing lines	Patch %	Lines
.../example/vu/android/empowerplant/MainFragment.java	0.00%	6 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #243   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         16      16           
  Lines        883     886    +3     
  Branches      67      67           
=====================================
- Misses       883     886    +3     

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