This PR introduces a new Autostop config, wait_for, which determines the condition to check for when resetting the idleness timer. This option works in conjunction with idle_minutes. This config is an enum:

1. "jobs_and_ssh" (default) - Wait for all jobs to complete AND all SSH sessions to disconnect, thus fixing [Core] SSH sessions should count as non-idle autostop #6292
2. "jobs" - Wait for all jobs to complete.
3. "none" - Stop immediately after idle time expires, regardless of running jobs or SSH connections.

With the introduction of "none" for doing hard stops, we should also rename idle_minutes, as it might be confusing for users. We can do that in a follow-up PR, along with making the field accept arbitrary time units (not only minutes). I've created #6382 to track that.

Context:
Today, idleness for autodown/stop is measured from the completion of all tasks in the cluster's queue. However, a user may be SSH'd in, for example to debug some stuff. So we should count SSH activity as non-idle.

This PR implements this by listing the contents of /dev/pts (the virtual filesystem where these pseudo-terminal devices reside) and counting them (excluding ptmx).

$ ls /dev/pts
0  1  2  3  ptmx

Initially, we considered using who, which is part of GNU coreutils. The command is simple, it just prints a list of users who are currently logged in. But we found out that SSH sessions from Cursor (probably VS Code too) did not show up under who (thanks @concretevitamin for reminding to test this path!), likely because their Remote-SSH extension does not spawn an interactive login process.

$ who
gcpuser  pts/0        2025-07-23 20:32 (12.121.71.90)
gcpuser  pts/1        2025-07-23 20:33 (12.121.71.90)

Another approach considered was:

$ ps ax | grep -v grep | grep "sshd:.*@"
   1655 ?        S      0:00 sshd: gcpuser@notty
   3669 ?        S      0:00 sshd: gcpuser@notty
   3977 ?        S      0:00 sshd: gcpuser@pts/0
   4074 ?        S      0:00 sshd: gcpuser@pts/1
   4160 ?        S      0:00 sshd: gcpuser@notty

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Documentations (for user-facing changes)
• Any manual or new tests for this PR (please specify below)
  - New smoke tests: test_autostop_wait_for_jobs_and_ssh, test_autostop_wait_for_none
  - Manually tested by ssh'ing from the terminal
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)