fix(native): Resolve the WER module relative to handler_path

[tustanivsky]

The native backend looks for sentry-wer.dll only next to the running executable. The crashpad backend resolves crashpad_wer.dll against handler_path instead, so an embedder that installs the handler elsewhere gets a working WER module on crashpad and a silent "Native WER module not found" on native.

In Unreal Engine plugin runtime dependencies are staged under the plugin's own Binaries directory, never next to the game executable:

Game/Binaries/Win64/Game.exe                        <- searched
Game/Plugins/sentry/Binaries/Win64/sentry-crash.exe <- handler_path
Game/Plugins/sentry/Binaries/Win64/sentry-wer.dll   <- ships with the handler

Since plugin doesn't place the DLL next to the executable WER-delivered crashes (__fastfail, heap corruption, stack buffer overrun) are lost on every native-backend build.

wer_module_path() now tries handler_path's directory first and falls back to the executable directory, matching crashpad. Existence checking moves into wer_path_in_dir() so the fallback is actually reached when the handler directory holds no module.

Related items:

• Add Windows Error Reporting support for the native backend sentry-unreal#1568

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.31%. Comparing base (394af78) to head (35d08c2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2073      +/-   ##
==========================================
- Coverage   74.41%   74.31%   -0.11%     
==========================================
  Files         103      103              
  Lines       26746    26761      +15     
  Branches     4873     4878       +5     
==========================================
- Hits        19903    19887      -16     
- Misses       5487     5527      +40     
+ Partials     1356     1347       -9     

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[jpnurmi]

Thanks!