[4.0] Update `ObservableQuery.getCurrentResult()` to return the last emitted result

[jerelmiller]

Currently ObservableQuery.getCurrentResult() returns a non-stable object when calling it multiple times. This is because it dynamically calculates the result, regardless of whether the value has actually been emitted or not (for example, its possible to get a loading state from getCurrentResult even when notifyOnNetworkStatusChange is false).

This makes it hostile to use in cases where you need a stable value. Furthermore it doesn't always reflect actual values emitted from the observable. We'd like to change this unpredictablility.

This change will also allow us to simplify our React hooks, such as useQuery, since it would allow us to use getCurrentResult with useSyncExternalStore without the need to track those state values externally.

[jerelmiller]

After playing around with this some more, I don't feel this change is necessary any longer. #12556 did most of what I was hoping for initially, which is providing a more stable result when called multiple times in a row.

My hope was that getCurrentResult or a function that returns just the last emitted result would be enough to refactor the use of useSyncExternalStore in useQuery/useLazyQuery, but this change didn't seem to reduce any code since we still need to know what the last value is inside the subscribe function to avoid rerendering the initial result multiple times. As such, I'm going to call this complete and suggest we leave what we have as-is.

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