Make regression pipeline test stacking deterministic

[niemyjski]

Summary

• generate the two setup events as logs so their distinct sources always produce separate stacks
• assert the setup stack IDs differ before exercising batch regression handling

Why

The default error-event generator randomly selects from a 25-error pool. When both setup events selected the same error, they shared a stack and the test incorrectly observed the first context as regressed.

Verification

• forced the two setup errors to match and reproduced Expected: Fixed / Actual: Regressed
• dotnet test tests/Exceptionless.Tests/Exceptionless.Tests.csproj -- --filter-class Exceptionless.Tests.Pipeline.EventPipelineTests (57 passed, 1 skipped)
• focused test passed 6 consecutive runs

Breaking changes

None.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Exceptionless.Insulation	37%	35%	286	❌
Exceptionless.Core	76%	67%	10280	✔
Exceptionless.Web	85%	69%	7974	✔
Exceptionless.AppHost	38%	41%	147	❌
Summary	78% (25509 / 32506)	68% (11960 / 17711)	18687	✔