Backport GetPreExistingChunksIdsAsync fix to data-ingestion-preview2 branch

Backport the fix from https://github.com/dotnet/extensions/pull/7311 into data-ingestion-preview2 branch

This is going to require updating the test, as IngestionChunk was extended with TokenCount property recently (https://github.com/dotnet/extensions/pull/7264)