Error messages and askpass logic differs from sudo.ws when no tty is available

[3v1n0]

Describe the bug

While working on #1591, I identified that sudo-rs behavior in PAM conversations without /dev/tty diverges from upstream sudo in both fallback selection and user-facing diagnostics.

In no-tty/headless scenarios, sudo-rs could fail or provide less actionable diagnostics where sudo still follows valid fallback paths.

This affects:

• Hidden password prompts (PromptEchoOff) when askpass is available.
• Non-hidden conversation flows when no tty is present.
• Display detection in Wayland sessions. (new as per tgetpass: Honor WAYLAND_DISPLAY and WAYLAND_SOCKET sudo-project/sudo#540)
• Remote SSH invocations without allocated tty (missing hint for ssh -t). (as per sudo-project/sudo@516f729)

To Reproduce

  ssh localhost sudo id
  SUDO_ASKPASS=/bin/askpass sudo true </dev/null

Expected behavior

sudo-rs should match sudo behavior for no-tty conversation handling and provide actionable, context-aware error messages.