[joshdaugherty]

Fixes #15548.

CollectionStructure::collection() caches the collection in Blink under collection-structure-collection-{handle}. Collection::save() clears collection-{handle}* by prefix, which doesn't match that key, and Collection::delete() doesn't clear it at all. So after a collection is replaced by a new instance with different sites, its structure keeps answering from the old instance, and structure()->in($site) returns null for a site the collection is now in.

This adds a Blink::forget() for that key to both save() and delete().

Tests:

• saving_clears_the_collection_memoized_by_its_structure saves a structured collection on en, resolves its structure, saves a new instance on fr, and asserts the structure resolves a tree for fr. It fails without the fix.
• deleting_clears_the_collection_memoized_by_its_structure asserts the key is gone after delete(). It fails without the fix.
• it_saves_the_collection_through_the_api mocks Blink strictly, so it now expects the extra forget().

Other ways to fix this, in case one of them is preferred:

• Rename the cache key to collection-{handle}-structure-collection, so the existing flushStartingWith() in save() covers it. That's cleaner, but delete() would still need an explicit clear.
• Stop caching the collection on the structure, and call Collection::findByHandle() each time. This runs once per entry, so it trades the cache for repeated repository lookups.

This is separate from #5046, which is about Tree's cached data and isn't changed here.