fix: count exception filter and exception processor failures as callback_error, and stop dropping events when an SDK processor fails

[jamescrosswell]

Follow-up to #5607, part of #5634. This covers items 3 and 4 there. They share a fix, so they're in one PR.

Summary

• Exception filters and exception processors get the same isolation as event processors. When an IExceptionFilter or ISentryEventExceptionProcessor throws, the SDK logs an error naming it, drops the event, and records callback_error. Before, the throw fell through to CaptureEvent's catch-all. That dropped the event, logged a generic message and counted nothing.
• A failing SDK processor no longer counts as the user's callback failing. SDK-owned event, exception and transaction processors implement a new internal ISdkProcessor marker. When one throws, the SDK logs the error and runs the remaining processors, as fix(core): [Callback Errors 3] Drop failed processor data sentry-java#6142 does. That's right for an SDK bug: the user's data isn't dropped, and the failure doesn't show up in their Stats as their own callback.

Notes for review

• Behaviour change: since 6.12.0, a throwing SDK processor dropped the event or transaction and recorded callback_error. Now it's sent, without whatever that processor would have added.
• The stack trace factory can be supplied by the user, and it's called from inside MainExceptionProcessor and MainSentryEventProcessor. Both calls now catch a failure there and log it. The event keeps its exception type, message and enrichment, and loses only the stack trace.
• DelegateEventProcessor and DelegateTransactionProcessor are deliberately not marked. They wrap the user's own lambdas (scope.AddEventProcessor(Func<…>)), and a test covers this.
• The two Entity Framework exception processors are public. Implementing an internal interface leaves their public API, and the approval snapshots, unchanged. A user subclass inherits the marker, as in Java.
• sentry-unity's own processors aren't marked. Sentry.Unity already has InternalsVisibleTo, so it can opt in later.

Closes #5684
Closes #5685

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.22%. Comparing base (8973c9b) to head (f2796cf).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5688      +/-   ##
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+ Coverage   75.21%   75.22%   +0.01%     
==========================================
  Files         515      516       +1     
  Lines       18989    19022      +33     
  Branches     3693     3698       +5     
==========================================
+ Hits        14282    14310      +28     
- Misses       3852     3857       +5     
  Partials      855      855              

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[jamescrosswell]

@ric-oliv looks like a lot to review but it's mostly just adding the marker attribute everywhere... then a few short lines checking for that marker attribute.