Thanks for the reminder! I think all relevant extensions are now listed at https://doc.wikimedia.org/cover-extensions/. If one is missing, I would add it first, because this makes it easier to see where @covers tags are missing.

In the past weeks I went through the "German technical wishlist" extensions (most notably Advanced-Search, Move-Files-To-Commons, as well as Two-Column-Edit-Conflict-Merge) and added @covers tags wherever appropriate, as well as added missing test cases.

Next steps, in my opinion:

• If an extension is missing at https://doc.wikimedia.org/cover-extensions/, add it. Create a separate ticket for this, if needed.
• If the coverage report for an extension looks bad, create a ticket for that specific extension, break it down, and bring it up in the responsible teams sprint planning. For FileImporter we just did this, see T190290.