Align callback error handling with other SDKs

### Description

Currently Java SDK does not follow the [new spec for callbacks](https://github.com/getsentry/sentry-docs/pull/19189).
If an error happens in a customer callback (hook), the SDK should fall back according to spec.

Gaps in Java SDK:
- `beforeBreadcrumb`: on error the breadcrumb is added as is, it should be dropped instead
- `eventProcessor`: on error the event/telemetry goes further in the pipeline, it should be dropped instead

Client reports:
- there's a new discard reason `callback_error ` (https://develop.sentry.dev/sdk/telemetry/client-reports/#discard-reasons-1) which should be used to make this distinguishable from things being filtered out on purpose.

Start by tracing the Java SDK handling for the beforeBreadcrumb and eventProcessor callbacks, then inspect how client-report discard reasons are represented. The work is complete when callback errors drop the breadcrumb or event and record the callback_error discard reason, matching the referenced callback specification.

Scritto dal modello di indicizzazione a partire dal testo della issue.