Prevent WebSockets from throwing during graceful shutdown

[halter73]

The same as #26914, but for 5.0.

This stops WebSockets from throwing as soon as Kestrel shutdown starts instead of after the ShutdownTimeout. This copies logic we already have in other places like Http1ContentLengthMessageBody.

Kestrel will call Transport.Input.CancelPendingRead() during shutdown so idle connections can wake up and shutdown gracefully. We manually call CancelPendingRead() to simulate this and ensure the Stream returned by UpgradeAsync doesn't throw in this case.

Addresses #26482 for 5.0

[ghost]

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[halter73]

Description

This prevents WebSockets from throwing during graceful shutdown.

Kestrel calls Transport.Input.CancelPendingRead() during shutdown so idle connections can wake up and close gracefully. This PR ignores canceled ReadResults in Streams returned by UpgradeAsync, so they don't throw early during shutdown.

Customer Impact

Without this fix, WebSocket read operations throw an OperationCanceledException at the beginning graceful shutdown (which lasts 5 seconds by default). WebSocket read operations should only throw if the graceful shutdown period expires to give customers the opportunity to use the WebSocket during the graceful shutdown period.

This was reported by a customer on GitHub. #26482

Regression?

No.

Risk

Low. It might seem a bit scary because we're adding a loop with a non-trivial exit condition, but we already have loops exactly like this in 3.1 (and 5.0 for that matter) in the more commonly used Http1ContentLengthMessageBody. See #26914 (comment).