Don't report SDK event processor failures as the user's callback_error #5685

Part of #5634.

Branch: main. No public API change, so this doesn't need to wait for v7.

SentryEventHelper.ProcessEvent (SentryEventHelper.cs:26-60) catches every event processor, the SDK's own included, and records callback_error when one throws. The hooks spec only requires isolating user-registered processors. A bug in an SDK processor would show up in the user's Stats as their callback failing.

getsentry/sentry-java#6162 marks SDK-owned processors to tell them apart, which is one way to do it.