test(library): pin collections, both as a library and as an item

"Is Collections broken too?" deserved an answer from the suite rather
than from reading the query.

Both, and they behave differently. A Collections *library* was hit by the
same defect as Books and Photos -- unmapped collection_type, no
include_item_types, so the library clause matched every cached row -- and
is fixed by the same change. The unknown-type test now covers boxsets,
photos, homevideos and the empty collection_type Jellyfin sends for a
mixed library, instead of standing on books alone.

An individual collection is a different path and keeps working: a
BoxSet's members carry parent_id, which the cache does store, so they
match the ordinary parent link rather than the library clause. That is
worth its own test because narrowing the clause could plausibly have
taken collections with it, and "Collections is empty" would look
identical to the bug being fixed.

docs/requirements.md

@@ -819,6 +819,7 @@ Internal architecture, components, and application logic.
 | UT-246 | Opening a PiP window disarms background audio, and a background signal arriving with the native PiP flag false is still treated as PiP while the frontend's latch says the window is open — without resurrecting one it has already seen close | DR-266 | Done |
 | UT-247 | A library whose `collection_type` has no mapping — Books, Photos, a mixed library — does not return the server's films, albums and shows from cache | DR-277 | Done |
 | UT-248 | Narrowing the library clause does not starve the libraries that do have landing pages: music, movies and TV each still list their own media and none of the others | DR-277 | Done |
+| UT-249 | Opening an individual collection still lists its own children: a BoxSet's members are matched by the stored `parent_id`, not by the library clause, so narrowing that clause did not empty collections | DR-277 | Done |
 ### Integration Tests
 
 | Test ID | Test Description | Traces To | Status |