Fix #57232 potential CancellationTokenSource leak in HubConnection

[AshkanAfsharpour]

Fix potential CancellationTokenSource leak in HubConnection.cs #57232

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Summary of the changes (Less than 80 chars)

Fixes potential memory leak by ensuring proper disposal of CancellationTokenSource in HubConnection.cs.

Description

This PR addresses issue #57232 regarding a potential memory leak in HubConnection.cs where a CancellationTokenSource is created but only Cancel() is called without disposing it. As per the discussion, I've refactored the code to use the using var pattern to ensure automatic disposal of the CancellationTokenSource.

Fixes #57232.

[AshkanAfsharpour]

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[BrennanConroy]

Thanks @AshkanAfsharpour