[release/6.0] Do not flush serializer buffers before first IAsyncEnumerable element is fetched

[github-actions]

Backport of #59808 to release/6.0

/cc @eiriktsarpalis

Customer Impact

The current STJ implementation of IAsyncEnumerable<T> serialization will attempt to flush the serializer's internal buffer to the underlying stream every time an IAsyncEnumerator.MoveNextAsync() call yields a running task. This is by design behavior, intended to echo the chunking strategy employed by the IAsyncEnumerable source.

Note however that many IAsyncEnumerable<T> implementations that wrap over network connections will delay establishing a connection until the first IAsyncEnumerator.MoveNextAsync() has been made. Thus in scenaria where establishing a connection has failed, the serializer will have already flushed data to the underlying stream, in a some cases preventing users from performing necessary error recovery. This is a customer reported issue.

This PR changes the buffer flushing logic so that the firstMoveNextAsync() does not prompt flushing of serializer buffers.

Testing

Unit testing has been added that validates the new behavior.

Risk

Low. Applies minor interventions to product code to modify the buffer flushing logic.

cc @pranavkm@davidfowl

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Issue Details

---

Backport of #59808 to release/6.0

/cc @eiriktsarpalis

Customer Impact

Testing

Risk

Author:	github-actions[bot]
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-