fix(database): prevent FUIArray crash on desynced child events

[demolaf]

Fixes #517.

FUIArray throws an uncatchable NSInternalInconsistencyException whenever a child event references a key missing from its local model — which legitimately happens with queryLimited(toLast:) under concurrent server-side writes, crashing the app inside the Firebase callback.

This guards the four throw sites to reconcile instead of crashing: removeSnapshot: no-ops when the key is already gone, insertSnapshot: appends on an unknown previous sibling, and changeSnapshot:/moveSnapshot: recover an unknown key as an insert. Adds a regression test per path.

Preview

Simulator.Screen.Recording.-.iPhone.17.Pro.-.2026-07-21.at.12.02.09.mov

[gemini-code-assist]

Code Review

This pull request addresses local model desynchronization issues (GitHub issue #517) in FUIArray by preventing crashes when operations encounter unknown keys. Instead of throwing exceptions, the code now gracefully handles these cases by appending or recovering them as inserts. Unit tests have been added to cover these scenarios. The feedback suggests making the new tests more robust by asserting the exact insertion index of recovered snapshots rather than just verifying the array count.