Fix readline backspace/line-wrapping on linux

[yorickvP]

termui contains a workaround for windows where it prints a readline prompt itself. On linux, this confuses readline so things like backspace/ctrl-backspace and line wrapping don't work correctly.

On platforms other than windows, just let readline handle the prompt.

fixes #2968

Note: while I used claude code to offload some manual work creating this PR, I've steered and reviewed all the changes and take personal responsibility for the contents.

[yorickvP]

Rebased out the conflicting version bumps and fixed the stdout/stderr situation.

[jan-swiecki]

I confirm that it resolves prompt backspace problems in llm (simonw/llm#356)

[kdeldycke]

I confirm this issue and the patch, which has been hinted at in #3021 and #3019. I noticed there the coupling of both prompt() / confirm() but could not make sense of their behavior. Until I saw this PR. This is the right fix but need to also apply to confirm(). I will amend this PR to fix both.

[kdeldycke]

@yorickvP : I just generalized your fix into a new _readline_prompt helper so both prompt() and confirm() benefits from it. Also squashed all the commit and rebased it on top of stable. Added some tests to confirm that <stdout> is no longer polluted.

[AndreasBackx]

Thanks again @yorickvP for these fixes. Also thank you @kdeldycke for the small cleanup and rebase.