fix SendTo with SocketAsyncEventArgs

[wfurt]

Fixes #97965 and #99863 -- a regression from PR #90086 (in 8.0).

With the new SocketAddress changes the task based IO worked because SendToAsync explicitly sets saea._socketAddress to null. But there is no access to it for anybody using SocketAsyncEventArgs directly.

In the past, RemoteEndPoint was only user accessible variable and _socketAddress was always managed internally. With the new API SendTo and ReceiveFrom can use SocketAddress directly and reference would be also set on SocketAsyncEventArgs. As I realized, 1) we should make no assumption about and and we should not try to use it for anything else than just the particular call and 2) SocketAsyncEventArgs can be used directly so any related logic really needs to be at the bottom and not in the layers above.

With that I made changes to set RemoteEndPoint to null to indicate that SendTo and ReceiveFrom was invoked with user provided SocketAddress. We will also detach it from SocketAsyncEventArgs in that case so it can be re-used for another operations safely.
On other cases we can keep it around and simply override _socketAddress as we see fit instead of allocating new one every time.

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Issue Details

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this fixes regression from #90086 and fixes #97965.

With the new SocketAddress changes the task based IO worked because SendToAsync explicitly sets saea._socketAddress to null. But there is no access to it for anybody using SocketAsyncEventArgs directly.

In the past, RemoteEndPoint was only user accessible variable and _socketAddress was always managed internally. With the new API SendTo and ReceiveFrom can use SocketAddress directly and reference would be also set on SocketAsyncEventArgs. As I realized, 1) we should make no assumption about and and we should not try to use it for anything else than just the particular call and 2) SocketAsyncEventArgs can be used directly so any related logic really needs to be at the bottom and not in the layers above.

With that I made changes to set RemoteEndPoint to null to indicate that SendTo and ReceiveFrom was invoked with user provided SocketAddress. We will also detach it from SocketAsyncEventArgs in that case so it can be re-used for another operations safely.
On other cases we can keep it around and simply override _socketAddress as we see fit instead of allocating new one every time.

Author:	wfurt
Assignees:	wfurt
Labels:	area-System.Net.Sockets
Milestone:	9.0.0

[stephentoub]

It's ok to null it here because it's only used synchronously as part of the send operation and not asynchronously?

[wfurt]

yes. We will make _socketAddress before first try. f we do not finish synchronously on Unix, it would be copied to queued *SendOperation. For Windows, the const sockaddr *lpTo is IN for WSASendTo and AFAIK it does not need to be preserved until the overlapped IO is completed.

[stephentoub]

The only two operations it would be set for are ReceiveFrom and SendTo, and we already handled the SendTo case on the synchronous start path?

[wfurt]

yes. For SendTo we clear it when operation starts, for ReceiveFrom we do it when the operation is finished.

[wfurt]

/backport to release/8.0-staging

[github-actions]

Started backporting to release/8.0-staging: https://github.com/dotnet/runtime/actions/runs/8268148999