DynamicCollection does not free the pages of old indexes on DropIndex and VacuumAsync(RebuildIndexes) #211

Version: main (efacaf4).

What happens

Typed collections free an index's pages when it is dropped or replaced (CollectionIndexManager.DropIndex, ReplaceIndex). DynamicCollection doesn't, for any index kind:

• DropIndex (DynamicCollection.cs:1737-1742) removes the index from the dictionary and calls PersistIndexMetadata(), but never frees the tree.
• VacuumAsync(RebuildIndexes = true) (:1511, persisted at :1615) builds fresh indexes and overwrites the entries, leaving every old tree unreachable.

Each drop and each rebuild therefore leaks a full tree. It also skips documents that ReadDocumentAt can't read without reporting them, unlike the typed rebuild since #170.

Expected

Same behaviour as typed collections: collect the old tree's pages (B-Tree, vector, spatial once #205 lands) and free them after the swap is persisted; on a failed rebuild, collect the copy's pages through the transaction before rolling back, as RebuildIndexFromScratchAsync does.