Fix PAM Auth for exec sessions with allocated TTY

[Joerger]

Changelog: Fix an issue in the Teleport SSH Service where interactive PAM Auth modules always fail when trying to run exec sessions with tty allocated. e.g. tsh ssh --tty <node> ls.

Fixes #61689

Manual Testing (8cd5232):

• Set up a custom interactive PAM auth module as described in PAM Auth with interactive input is broken. #49028.
• tsh ssh --tty <node> ls prompts for authentication and succeeds.
• tsh ssh <node> prompts for authentication and succeeds.
• Starting an interactive session in the WebUI prompts for authentication and succeeds.

Note: tsh ssh <node> ls without -tty and with interactive PAM auth fails both before and after this PR, as expected. Unfortunately, this failure manifests as a stalled connection which eventually times out or is canceled by the user, and it's unclear whether this is improvable within this boundaries of PAM. Either way, that issue is out of scope of this PR.

[rosstimothy]

@Joerger can we also cover that tsh ssh <node> ls and the Web UI still behave as expected in the test plan?

[Joerger]

@Joerger can we also cover that tsh ssh <node> ls and the Web UI still behave as expected in the test plan?

Added a manual test for the WebUI. tsh ssh ls without --tty is not expected to work currently, so I just added a note in the PR description.

[rosstimothy]

Note: tsh ssh <node> ls without -tty and with interactive PAM auth fails both before and after this PR, as expected. Unfortunately, this failure manifests as a stalled connection which eventually times out or is canceled by the user, and it's unclear whether this is improvable within this boundaries of PAM. Either way, that issue is out of scope of this PR.

Do we have an open issue for this? Should we create one if not?

[Joerger]

@eriktate Friendly ping to review