search pagination: define behavior for cloning/missing repositories

Created by: slimsag

This PR defines the behavior of the search pagination API for cloning/missing repositories that are encountered during a paginated search request (as well as repos, searched, indexed, and timedout but these are less interesting / important).

In the future, I would like to investigate ways to make the logic here more legible (i.e. simpler) but this will require larger overhauls than I could reasonably make in this PR. In the meantime, this is a clear improvement over the prior (undefined, very arbitrary) behavior.

Fixes #6000 (closed)

Test plan: Extensive tests added via TDD.