Handle DefaultConnectionContext abort/dispose cancellation race

[scarab-systems]

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Summary of the changes (Less than 80 chars)

Handle DefaultConnectionContext abort/dispose cancellation race

Description

DefaultConnectionContext.Abort() queues cancellation of the connection-closed token on the ThreadPool. If the connection is disposed before that queued work runs, the queued Cancel() can observe a disposed CancellationTokenSource and raise ObjectDisposedException on the background worker.

This change makes the queued cancellation tolerate that already-disposed state. It also adds a RemoteExecutor regression test that forces the queued cancellation to run after DisposeAsync().

Validation:

• ./restore.sh --no-build-nodejs --no-build-java --no-build-native
• source ./activate.sh >/dev/null && dotnet test src/Servers/Kestrel/Core/test/Microsoft.AspNetCore.Server.Kestrel.Core.Tests.csproj --no-restore --filter DefaultConnectionContextDisposeAsyncAfterAbortDoesNotCrashProcess
• source ./activate.sh >/dev/null && dotnet test src/Servers/Kestrel/Core/test/Microsoft.AspNetCore.Server.Kestrel.Core.Tests.csproj --no-restore

Fixes #57098

[dotnet-policy-service]

Thanks for your PR, @scarab-systems. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[scarab-systems]

@dotnet-policy-service agree