fix: check file-writer construction when writing envelope to path

[supervacuus]

fixes #1035.

• Fixed two other missing path checks and an incorrect file-writer loop on Windows.
• Added convenience powershell scripts for local test runs on Windows.
• Clarified platform support of sentry_handle_exception() as requested here: https://github.com/getsentry/sentry-native/issues/1032#issuecomment-2312105216

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.78%. Comparing base (a762b23) to head (e9b664a). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1036 +/- ## ========================================== + Coverage 83.75% 83.78% +0.02% ========================================== Files 53 53 Lines 5510 5512 +2 Branches 1197 1198 +1 ========================================== + Hits 4615 4618 +3 + Misses 782 781 -1 Partials 113 113 ```