nixos/test-driver: deprecate shell_interact()

Reviewing #512771 got me thinking and I'm not sure I see a good reason
to keep this method given the SSH backdoor functionality has a much
better UX.

I'd propose we deprecate it, await 26.11 to see if people complain and
remove it alltogether after that.

nixos/doc/manual/development/running-nixos-tests-interactively.section.md

@@ -43,6 +43,11 @@ test script).
 
 ## Shell access to VMs in interactive mode {#sec-nixos-test-shell-access}
 
+::: {.warning}
+Using `shell_interact()` is deprecated. Use the
+[interactive SSH backdoor](#sec-nixos-test-ssh-access) instead.
+:::
+
 The function `<yourmachine>.shell_interact()` grants access to a shell running
 inside a virtual machine. To use it, replace `<yourmachine>` with the name of a
 virtual machine defined in the test, for example: `machine.shell_interact()`.

nixos/doc/manual/release-notes/rl-2611.section.md

@@ -56,6 +56,8 @@
 
 - `komodo` has been updated to the v2 release line (2.x). See the [upstream v1 → v2 upgrade guide](https://github.com/moghtech/komodo/releases/tag/v2.0.0).
 
+- The `shell_interact()` function on interactive runs of NixOS VM tests has been deprecated. Use the SSH backdoor instead.
+
 - `security.run0.enableSudoAlias` now uses the `run0-sudo-shim` instead of a shell-script to improve compatibility.
 
 - `security.run0.persistentAuth` options have been added to support persistent Authentication of session. Timeout configurable via `security.polkit.settings.Polkitd.ExpirationSeconds`.

nixos/lib/test-driver/src/test_driver/machine/__init__.py

@@ -908,6 +908,7 @@ class QemuMachine(BaseMachine):
 
         return (rc, output.decode(errors="replace"))
 
+    @warnings.deprecated("Use the SSH backdoor instead")
     def shell_interact(self, address: str | None = None) -> None:
         """
         Allows you to directly interact with the guest shell. This should