fix(io): finalize TCP transport cleanup before actor stop

[Aaronontheweb]

Summary

This is a follow-up to @to11mtm's comment on #8132 about async transport disposal in the TCP actor lifecycle:
#8132 (comment)

This change makes the graceful shutdown path explicit instead of relying on PostStop() fallback cleanup.

• finalize ITransportConnection asynchronously during Tcp.Close
• finalize ITransportConnection after both FINs during Tcp.ConfirmedClose
• keep PostStop() abort logic as the fallback for unexpected actor termination
• make TcpTransportConnection cleanup idempotent across ShutdownAsync, CloseAsync, Abort, and DisposeAsync

Why

TcpTransportConnection already exposed DisposeAsync, but the actor lifecycle never actually used it. In practice that meant:

• graceful close paths could still fall through to abort-style cleanup in PostStop()
• ConfirmedClose did not have an explicit async finalization step after the close handshake
• the transport cleanup contract was harder to reason about across multiple shutdown paths

This PR makes the intended lifecycle explicit and keeps the emergency-stop behavior unchanged.

Testing

• added focused TcpConnectionBatchingSpec coverage for:
  • graceful close uses async finalization
  • confirmed close uses async finalization after peer FIN
  • unexpected stop still falls back to abort cleanup
• dotnet test src/core/Akka.Tests/Akka.Tests.csproj -c Release --filter \"FullyQualifiedName~TcpConnectionBatchingSpec|FullyQualifiedName~TcpIntegrationSpec\"
• dotnet build src/core/Akka/Akka.csproj -c Release
• dotnet test src/core/Akka.API.Tests/Akka.API.Tests.csproj -c Release