chore: Add integration tests for Windows and Linux

[tustanivsky]

This PR adds integration tests for the Sentry Unreal SDK using PowerShell's Pester framework. The tests validate end-to-end functionality by running the SentryPlayground sample application and verifying that events are properly captured and sent to Sentry.

Key Changes

Integration Test Framework:

• PowerShell/Pester-based integration tests (integration-test/) that execute the pre-built sample app.
• Cover crash capture and message capture scenarios.
• Validate event data structure, user context, tags, breadcrumbs, and stack traces.
• Use CMake to fetch dependencies (e.g., app-runner module for Sentry API integration).
• Includes a comprehensive README with setup and usage instructions.

CI/CD Integration:

• New reusable workflows for Linux and Windows integration tests.
• Run integration tests after unit tests across multiple Unreal Engine versions.
• Upload test outputs and logs on failure to assist with debugging.

Sample App Enhancements

• Improved log and events flushing to ensure output is captured
• Workarounds for platform-specific issues (log duplication on Linux UE 4.27)

Important notices:

• On Windows, integration tests run only for UE 5.2 and newer as that’s the first version where disabling the default Crash Reporter became possible. This allows crashes to be captured using the Sentry integration.
• Running the test app pre-built with UE 5.2 or 5.3 on Windows GitHub runners requires the DirectX June 2010 Runtime to be installed (this is also included in the official Epic Games Windows runtime Docker images)

Enabled by:

• chore: Update sample project to enable running integration tests in CI #1064
• fix: handle JSON with empty string keys in API responses app-runner#12

Related issues:

• CI Pipeline Improvements for Cross-Platform Support #1002

Closes:

• Integration/smoke test #77
• Improve CI - test on more versions, install final package for validation, test on more linux distros #575

#skip-changelog

[mujacica]

Do we also want to test logs, non-fatal event (ensure) and/or attachments too?

[tustanivsky]

Yes, we can cover more of the plugin’s features with tests in subsequent PRs. This one is primarily focused on introducing the testing framework itself and covering the basics.