Count `callback_error` for native Android events when `BeforeSend` throws

[jamescrosswell]

Part of #5634. Blocked by getsentry/sentry-java#6141, and a Java SDK bump to the release that ships it.

Branch: main. No public API change, and getsentry/sentry-java#6141 targets sentry-java 8.x, so the fix arrives with a regular Java SDK bump.

When the user's BeforeSend throws on a native Android event, #5610's BeforeSendWrapper returns null (Platforms/Android/SentrySdk.cs:246-255). sentry-java then counts before_send / error (SentryClient.java:171-177). sentry-java 8.58.0 has no callback_error reason yet (DiscardReason.java).

getsentry/sentry-java#6141 (part of getsentry/sentry-java#6081) adds DiscardReason.CALLBACK_ERROR and records it when a beforeSend* callback throws. sentry-java already drops the event on a throw; it just counts it as before_send today.

To investigate once it ships

Instead of returning null, BeforeSendCallback.Execute could throw a Java exception. sentry-java would then drop the event and count callback_error itself. This depends on how .NET for Android marshals managed exceptions across JNI, which needs checking on a device.

[linear-code]

NET-844