Reduce TCP channel async wait allocations

[Copilot]

• Update IMessageSocket: remove bool Send(IMessageSocketAsyncEventArgs), MessageSocketEventArgs(), void ReadNextMessage(); add ValueTask SendAsync(ReadOnlyMemory<byte>, CancellationToken), ValueTask SendAsync(IList<ArraySegment<byte>>, CancellationToken), Task ReadNextMessageAsync(CancellationToken)
• Rewrite TcpMessageSocket: replace SocketAsyncEventArgs-based state machine with Socket.ReceiveAsync(Memory<byte>) async read loop and Socket.SendAsync(ReadOnlyMemory<byte>) writes (with .NET Framework fallback via TaskCompletionSource + SocketAsyncEventArgs)
• Refactor UaSCBinaryChannel.BeginWriteMessage: remove OnWriteComplete, replace with fire-and-forget SendAsync + CompleteWriteAsync continuation helper
• Update all ReadNextMessage() call sites in TcpListenerChannel, TcpServerChannel, UaSCBinaryClientChannel → Socket.ReadNextMessageAsync()
• Build clean: 0 warnings, 0 errors
• All 6639 core tests pass
• Code review: no issues
• CodeQL: no alerts

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[codecov]

Codecov Report

❌ Patch coverage is 60.90909% with 86 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.95%. Comparing base (df24255) to head (10f7f7f).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
Stack/Opc.Ua.Core/Stack/Tcp/TcpMessageSocket.cs	57.89%	38 Missing and 10 partials ⚠️
Stack/Opc.Ua.Core/Stack/Tcp/UaSCBinaryChannel.cs	40.90%	21 Missing and 5 partials ⚠️
...ack/Opc.Ua.Core/Stack/Tcp/ChannelAsyncOperation.cs	80.00%	9 Missing and 2 partials ⚠️
...k/Opc.Ua.Core/Stack/Tcp/UaSCBinaryClientChannel.cs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3794      +/-   ##
==========================================
- Coverage   72.00%   71.95%   -0.05%     
==========================================
  Files         677      677              
  Lines      130752   130745       -7     
  Branches    22261    22254       -7     
==========================================
- Hits        94143    94079      -64     
- Misses      29946    29994      +48     
- Partials     6663     6672       +9     

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