Handle master pty allocation failure

[salva]

Summary

Fail master startup immediately when IO::Pty->new cannot allocate a pseudo-tty.

Changes

• Check the return value from IO::Pty->new in _master_start.
• Report a clear master failure instead of continuing without the pty and later producing a generic connection/login failure.

Fixes #42.

Testing

• perl -Ilib -c lib/Net/OpenSSH.pm
• perl -Ilib t/1_run.t

[Copilot]

Pull request overview

This PR improves failure handling during SSH master startup by immediately aborting when a requested pseudo-tty (pty) cannot be allocated, preserving the real root-cause error instead of allowing startup to proceed and fail later with a generic message.

Changes:

• Add an explicit success check after IO::Pty->new in _master_start.
• Fail master startup early via _master_fail with a clear “unable to allocate pseudo-tty” error message (including $!).

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