fix: Don't set current thread when one is already there

[bitsandfoxes]

The Problem

The Sentry SDK for Unity receives stringified stack traces directly from Unity's logging system for error events (see getsentry/sentry-unity#2377). After parsing these stack traces, the Unity SDK sets them on the event as the current thread before passing the event to the .NET SDK for processing.

However, MainSentryEventProcessor is unconditionally adding a new "current thread" (with Current = true) whenever AttachStacktrace was enabled and no exception stack trace was present.

sentry-dotnet/src/Sentry/Internal/MainSentryEventProcessor.cs

Lines 94 to 101 in 396a7ec

	var thread = new SentryThread
	{
	Crashed = false,
	Current = true,
	Name = currentThread.Name,
	Id = currentThread.ManagedThreadId,
	Stacktrace = stackTrace
	};

This resulted in events having two threads marked as current - the one from Unity with the correct stack trace, and a duplicate one captured by the .NET SDK.

Proposal

Just skip automatic current thread creation if the event already contains a thread with Current = true. This is not even related to the AttachStackTrace option as there only ever is one current thread anyway.

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (version6@8561566). Learn more about missing BASE report.

Additional details and impacted files

@@             Coverage Diff             @@
##             version6    #4681   +/-   ##
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  Coverage            ?   73.18%           
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  Files               ?      480           
  Lines               ?    17421           
  Branches            ?     3437           
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  Hits                ?    12749           
  Misses              ?     3821           
  Partials            ?      851           

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[jamescrosswell]

Generally looks good. I suggested one minor perf tweak...