Enforce etag Concurrency in Data Exlporer

Would be nice if a document was edited by another party or process while editing it, that upon hitting save, you received an error, even if this was an opt-in feature.

Similar to the etag concurrency that EF core users, but for the data explorer.

Start by reviewing the Data Explorer document editing and save flow, then identify how Cosmos DB document updates expose etag concurrency. Define the opt-in behavior and verify that saving after an external edit reports an error instead of silently overwriting the document.

Written by the indexing model from the issue text.