xdg-user-dirs: warn on setSessionVariables default change

Switch xdg.userDirs.setSessionVariables to the shared state-version default helper so older configurations get an explicit warning before the legacy default is removed.

Add focused tests for the legacy and current branches to verify that the session variable exports remain enabled before 26.05 and are disabled by default starting at 26.05.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

modules/misc/xdg-user-dirs.nix

@@ -136,10 +136,21 @@ in
 
     setSessionVariables = mkOption {
       type = with types; bool;
-      default = lib.versionOlder config.home.stateVersion "26.05";
-      defaultText = literalExpression ''
-        lib.versionOlder config.home.stateVersion "26.05"
-      '';
+      inherit
+        (lib.hm.deprecations.mkStateVersionOptionDefault {
+          inherit (config.home) stateVersion;
+          since = "26.05";
+          optionPath = [
+            "xdg"
+            "userDirs"
+            "setSessionVariables"
+          ];
+          legacy.value = true;
+          current.value = false;
+        })
+        default
+        defaultText
+        ;
       description = ''
         Whether to set the XDG user dir environment variables, like
         `XDG_DESKTOP_DIR`.

tests/modules/misc/xdg/default.nix

@@ -7,4 +7,6 @@
   xdg-user-dirs-mixed = ./user-dirs-mixed.nix;
   xdg-user-dirs-null = ./user-dirs-null.nix;
   xdg-user-dirs-short = ./user-dirs-short.nix;
+  xdg-user-dirs-session-vars-legacy = ./user-dirs-session-vars-legacy.nix;
+  xdg-user-dirs-session-vars-current = ./user-dirs-session-vars-current.nix;
 }

tests/modules/misc/xdg/user-dirs-session-vars-current.nix

@@ -0,0 +1,10 @@
+{
+  home.stateVersion = "26.05";
+
+  xdg.userDirs.enable = true;
+
+  nmt.script = ''
+    assertFileNotRegex home-path/etc/profile.d/hm-session-vars.sh \
+      '^export XDG_DESKTOP_DIR='
+  '';
+}

tests/modules/misc/xdg/user-dirs-session-vars-legacy.nix

@@ -0,0 +1,10 @@
+{
+  home.stateVersion = "25.11";
+
+  xdg.userDirs.enable = true;
+
+  nmt.script = ''
+    assertFileContains home-path/etc/profile.d/hm-session-vars.sh \
+      'export XDG_DESKTOP_DIR="/home/hm-user/Desktop"'
+  '';
+}