lib/evalNixvim: deprecate specialArgs.helpers

This has been soft-deprecated for several releases. Now that we have
removed our internal usage, we can introduce an eval warning.

lib/modules.nix

@@ -48,8 +48,12 @@ in
         # however see https://github.com/nix-community/nixvim/issues/2879
         inherit lib;
         modulesPath = ../modules;
-        # TODO: deprecate `helpers`
-        helpers = self;
+        # TODO: deprecated 2025-11-19
+        helpers = lib.warn ''
+          nixvim: the `helpers` module arg has been renamed to `lib.nixvim`.
+          Nixvim modules can access this via the `lib` module arg.
+          For wrapper modules (e.g. NixOS or Home Manager modules), see:
+          https://nix-community.github.io/nixvim/lib/nixvim/index.html#accessing-nixvims-functions'' self;
       }
       // extraSpecialArgs;
     };

tests/enable-except-in-tests.nix

@@ -20,6 +20,7 @@ let
             message = "Expected lib.nixvim.enableExceptInTests to be true";
           }
           {
+            # NOTE: evaluating `helpers` here prints an eval warning
             assertion = !helpers.enableExceptInTests;
             message = "Expected helpers.enableExceptInTests to be false";
           }
@@ -40,6 +41,7 @@ let
                 message = "Expected lib.nixvim.enableExceptInTests to be true";
               }
               {
+                # NOTE: evaluating `helpers` here prints an eval warning
                 assertion = helpers.enableExceptInTests;
                 message = "Expected helpers.enableExceptInTests to be true";
               }

tests/test-sources/extended-lib.nix

@@ -8,6 +8,7 @@ let
           message = "lib.nixvim should be defined";
         }
         {
+          # NOTE: evaluating `helpers` here prints an eval warning
           assertion = builtins.attrNames lib.nixvim == builtins.attrNames helpers;
           message = "lib.nixvim and helpers should be aliases";
         }