Add cache-first regression test for Storage.multiSet rejection

Adds a parallel test inside the 'mergeCollection cache-first ordering'
describe asserting that cache + subscribers reflect the merged data
even when Storage.multiSet (the new-keys path) rejects. The existing
test only exercises the Storage.multiMerge (existing-keys) path.

Also adds an afterEach that restores StorageMock.multiMerge and
StorageMock.multiSet to their originals after each test in this
block, so the rejecting mocks no longer leak into later describe
blocks (eviction, afterInit) whose setup relies on these storage
methods working normally.

Addresses review feedback on Expensify#787.

Author: elirangoshen

tests/unit/onyxUtilsTest.ts

@@ -805,6 +805,18 @@ describe('OnyxUtils', () => {
     });
 
     describe('mergeCollection cache-first ordering', () => {
+        // Save originals so we can restore them after each test. The tests below replace
+        // StorageMock.multiMerge / StorageMock.multiSet with rejecting mocks; without
+        // restoring, the mock leaks into later describe blocks (e.g. eviction tests) whose
+        // setup relies on these storage methods working normally.
+        const originalMultiMerge = StorageMock.multiMerge;
+        const originalMultiSet = StorageMock.multiSet;
+
+        afterEach(() => {
+            StorageMock.multiMerge = originalMultiMerge;
+            StorageMock.multiSet = originalMultiSet;
+        });
+
         it('updates cache and notifies subscribers even when Storage.multiMerge rejects', async () => {
             const collectionKey = ONYXKEYS.COLLECTION.TEST_KEY;
             const existingMemberKey = `${collectionKey}1`;
@@ -845,6 +857,46 @@ describe('OnyxUtils', () => {
             expect(lastBroadcast?.[existingMemberKey]).toEqual({value: 'merged'});
             expect(lastBroadcast?.[newMemberKey]).toEqual({value: 'new'});
         });
+
+        it('updates cache and notifies subscribers even when Storage.multiSet rejects', async () => {
+            const collectionKey = ONYXKEYS.COLLECTION.TEST_KEY;
+            const newMemberKey1 = `${collectionKey}1`;
+            const newMemberKey2 = `${collectionKey}2`;
+
+            // No keys are seeded, so every merged key is a "new" key. This forces the merge path
+            // to use Storage.multiSet (existing keys would go through Storage.multiMerge).
+            const collectionCallback = jest.fn();
+            Onyx.connect({
+                key: collectionKey,
+                waitForCollectionCallback: true,
+                callback: collectionCallback,
+            });
+            await waitForPromisesToResolve();
+            collectionCallback.mockClear();
+
+            // Force Storage.multiSet to reject with a non-retriable IDB error so the failure
+            // path is taken without burning the full retry budget and without rejecting the
+            // outer Onyx.mergeCollection promise.
+            const nonRetriableIdbError = Object.assign(new Error('Internal error opening backing store for indexedDB.open.'), {name: 'UnknownError'});
+            StorageMock.multiSet = jest.fn().mockRejectedValue(nonRetriableIdbError);
+
+            await Onyx.mergeCollection(collectionKey, {
+                [newMemberKey1]: {value: 'first'},
+                [newMemberKey2]: {value: 'second'},
+            } as GenericCollection);
+
+            // Cache must reflect the merge regardless of the multiSet rejection. This is the
+            // cache-first / storage-second invariant that mergeCollectionWithPatches must honor.
+            const cachedCollection = OnyxCache.getCollectionData(collectionKey);
+            expect(cachedCollection?.[newMemberKey1]).toEqual({value: 'first'});
+            expect(cachedCollection?.[newMemberKey2]).toEqual({value: 'second'});
+
+            // Subscribers must have been notified with the merged values.
+            expect(collectionCallback).toHaveBeenCalled();
+            const lastBroadcast = collectionCallback.mock.calls.at(-1)?.[0] as Record<string, unknown> | undefined;
+            expect(lastBroadcast?.[newMemberKey1]).toEqual({value: 'first'});
+            expect(lastBroadcast?.[newMemberKey2]).toEqual({value: 'second'});
+        });
     });
 
     describe('storage eviction', () => {