kubectl exec escape sequence (tilde-dot)

As a big user of `kubectl exec` _and_ a big commuter to and from work, I often am in situations where I have some `exec` instance open, but the connection is flaky.

When that happens, I _know_ that the connection will eventually timeout, but until then, that terminal window has become useless.

ssh has a nice `~.` (tilde-dot) escape sequence that instantly terminates the connection in those situations.

Perhaps `kubectl` can use the same technique? For someone like me, that would be a great UX improvement.

Start at the kubectl exec entry point and trace how terminal input and the remote connection are handled. Define the tilde-dot behavior and verify that it terminates a flaky exec session without affecting ordinary input; add focused coverage for the escape sequence and run the relevant kubectl exec tests.

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