Cleanup usage of OrphanBuffer

The interface `OrphanBuffer` and its implementation `DefaultOrphanBuffer` are used in multiple tests. However, the current implementation of `OrphanBuffer` is an implementation detail that we want to replace easily.

We should check:

- Do these tests really need a full implementation of the `OrphanBuffer`, or can we replace it?
- Can we replace the OrphanBuffer with the whole event intake pipeline? This would also prevent the exposure of internal details.
- Can we limit the usage of the `OrphanBuffer` to a few places that we explicitly export to?
- Are there other solutions that allow us to move `OrphanBuffer` and `DefaultOrphanBuffer` into `event-intake-impl`?

Search the tests and production code for usages of OrphanBuffer and DefaultOrphanBuffer, then inspect the event intake pipeline and the event-intake-impl boundary. Determine whether tests can use the pipeline or a narrower exported interface, and document a selected approach that permits both classes to move into event-intake-impl with no unintended exposure.

Written by the indexing model from the issue text.