fix: allow tui to open /dev/tty as fallback

[jedevc]

Some various little things found while investigating TTY magic in the progress output. Original discussion in discord.

It seems like we'll also need a bubbletea fix, as noticed by @vito:

https://github.com/charmbracelet/bubbletea/blob/4ad07926d7ff00bc21a05b2536d82a7cc629225e/tea.go#L738-L740 it does this unconditionally vs. https://github.com/charmbracelet/bubbletea/blob/4ad07926d7ff00bc21a05b2536d82a7cc629225e/tea.go#L582-L586

However, we should be able to cleverly fallback to /dev/tty if it exists, which is what this PR does.

You can simulate /dev/tty not existing by using the setsid wrapper (however, this still breaks until the above-mentioned bubbletea fix can be made). E.g.:

❯ cat /dev/tty
...
❯ setsid cat /dev/tty
cat: /dev/tty: No such device or address

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fix: avoid panic if no stdin is available when launching terminal

If it's available, we should try to fallback to /dev/tty as a fallback.
This logic is borrowed from bubbletea, however, we can't just use
WithInputTTY, since this requires that /dev/tty exists - we want to
continue on, even if it doesn't.

This allows us to neatly handle the echo "" | dagger shell case -
stdin is not a TUI, but we can still usually find one at /dev/tty, so
we should try and do that.

Note: we also do a little refactor of findTTYs to directly return
io.Reader/io.Writers - this avoids the horror of typed nils, which
can result in passing a nil os.File into bubbletea (which is does
not like).

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fix: avoid panic if no stdin is available when launching terminal

This really doesn't happen except in truly bizarrely concocted test
cases - it's just nice to not panic if it does happen.