Guard against negative array size in MainActivity

[sentry]

This pull request was triggered by a Seer handoff from Sentry.

Root cause: The click listener for the negative_index button in MainActivity allocated new int[-5] directly. Java rejects negative array sizes, so every tap threw an unhandled NegativeArraySizeException and crashed the app.

Solution: The requested array size is now held in a variable and validated before the allocation. If it is negative, the listener reports an IllegalArgumentException to Sentry as a handled event (consistent with the other handled-error buttons) and returns without allocating, so the app no longer crashes.

Fixes ANDROID-NG

[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 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (21c77a4) to head (a15724e).

Files with missing lines	Patch %	Lines
...main/java/com/example/vu/android/MainActivity.java	0.00%	5 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #273   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         16      16           
  Lines        883     887    +4     
  Branches      67      68    +1     
=====================================
- Misses       883     887    +4     

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