[release/6.0] Do not flush serializer buffers before first IAsyncEnumerable element is fetched#59865

Anipik merged 3 commits intorelease/6.0dotnet/runtime:release/6.0from

backport/pr-59808-to-release/6.0dotnet/runtime:backport/pr-59808-to-release/6.0Copy head branch name to clipboard