FeedRange: Fix split handling for readfeed split

[ealsur]

Description

ReadFeed operations go through the PartitionKeyRangeHandler to resolve and the continuation token gets wrapped into a CompositeContinuationToken: https://github.com/Azure/azure-cosmos-dotnet-v3/blob/master/Microsoft.Azure.Cosmos/src/Handler/PartitionKeyRangeHandler.cs#L132

This wrapping only affects Document ReadFeeds because Change Feed does not use CT (but rather ETag).

FeedRangeContinuation handles splits by creating a list of CompositeContinuationTokens for the child partitions and copying the parent's continuation to them.

For ReadFeed the problem is that if we just simply copy the token to the children, the Token has the original range that got processed by the PartitionKeyRangeHandler and when the next request happens and it sends the parent's CT, the PartitionKeyRangeHandler will try and honor the CT's Range instead of the FeedRange: https://github.com/Azure/azure-cosmos-dotnet-v3/blob/master/Microsoft.Azure.Cosmos/src/Handler/PartitionKeyRangeHandler.cs#L94

This PR handles the wrapping of the PartitionKeyRangeHandler conditionally during a split-handling.

Type of change

• Bug fix (non-breaking change which fixes an issue)

[kirankumarkolli]

Please check comments

[j82w]

Suggested change

	public override bool IsDone => this.CompositeContinuationTokens.Count == 0;
	public override bool IsDone => this.CompositeContinuationTokens.Any();

[ealsur]

No, this would do the opposite. Done is if there are no more items in the queue, the change with Any would return true if there is at least one.

[j82w]

What happens to the other composite continuation tokens? Shouldn't this output a list of composite continuation tokens or should it be renamed to TryParseFirstCompositeContinuationToken?

[ealsur]

This is the same logic that exists in PartitionKeyRangeHandler, the list is a backward compatible measure, it contains 1 item

[j82w]

Suggested change

	this.CurrentToken = this.CompositeContinuationTokens.Count > 0 ? this.CompositeContinuationTokens.Peek() : null;
	if(!this.CompositeContinuationTokens.TryPeek(out this.CurrentToken)){
	this.CurrentToken = null;
	}

[ealsur]

I am tempted to pick this change, but I already know the current state passes all expected validations. I would need to validate splits on master all over with any code changes and this is mainly a readability optimization