agent-host: Cancel SSH connect when auth prompt is dismissed (#316785)

* Cancel SSH connect on keyboard-interactive dismissal (Written by Copilot)

Treating a dismissed keyboard-interactive prompt as empty responses leaves the SSH connect promise waiting for the auth flow to time out, so the sessions connection progress notification can spin indefinitely. Cancel the owning connect attempt instead and suppress the expected cancellation error in the remote connection action. (Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Ensure SSH auth cancellation wins (Written by Copilot)

Settle the SSH connect promise as cancellation before unblocking ssh2 so synchronous auth failures cannot surface as user-visible connection failures. Add regression coverage through the _connectSSH flow. (Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>