helix: assert languages migration warning

tests/modules/programs/helix/default.nix

@@ -1,4 +1,5 @@
 {
   helix-example-settings = ./example-settings.nix;
+  helix-legacy-languages = ./legacy-languages.nix;
   helix-only-extraconfig = ./only-extraconfig.nix;
 }

tests/modules/programs/helix/legacy-languages.nix

@@ -0,0 +1,57 @@
+{
+  config,
+  pkgs,
+  ...
+}:
+
+{
+  # Exercise merging legacy list definitions with new attrset definitions.
+  imports = [
+    {
+      programs.helix.languages.language-server.nil = {
+        command = "nil";
+      };
+    }
+  ];
+
+  programs.helix = {
+    enable = true;
+    package = config.lib.test.mkStubPackage { outPath = "@helix@"; };
+    languages = [
+      {
+        name = "rust";
+        auto-format = false;
+      }
+    ];
+  };
+
+  test.asserts.warnings.expected = [
+    ''
+      Using `programs.helix.languages` as a list is deprecated and will be
+      removed in a future release. Please use `programs.helix.languages.language` instead.
+
+      This option now generates the whole languages.toml file instead of just the language array in that file.
+
+      Use:
+        programs.helix.languages = { language = <languages list>; }
+
+    ''
+  ];
+
+  nmt.script =
+    let
+      expectedLanguages = pkgs.writeText "helix-languages.expected.toml" ''
+        [[language]]
+        auto-format = false
+        name = "rust"
+
+        [language-server.nil]
+        command = "nil"
+      '';
+    in
+    ''
+      assertFileContent \
+        home-files/.config/helix/languages.toml \
+        ${expectedLanguages}
+    '';
+}