test: add tests for function equality behavior

Add tests for function equality covering both direct comparisons and
comparisons within composite types (lists and attribute sets).

Tests verify:
- Direct function comparisons always return false
- Value identity optimization in composite types allows identical
  functions to compare as equal when both references point to the
  same function value

tests/functional/lang/eval-okay-equal-function-attrset-distinct-similar.exp

@@ -0,0 +1 @@
+false

tests/functional/lang/eval-okay-equal-function-attrset-distinct-similar.nix

@@ -0,0 +1,3 @@
+# Distinct but not identical functions in attribute set compare as unequal
+# See https://nix.dev/manual/nix/latest/language/operators#equality
+{ a = (x: x); } == { a = (x: x); }

tests/functional/lang/eval-okay-equal-function-attrset-identical.exp

@@ -0,0 +1 @@
+true

tests/functional/lang/eval-okay-equal-function-attrset-identical.nix

@@ -0,0 +1,10 @@
+# Function comparison in attribute set uses value identity optimization
+# See https://nix.dev/manual/nix/latest/language/operators#value-identity-optimization
+let
+  f = x: x;
+in
+{
+  a = f;
+} == {
+  a = f;
+}

tests/functional/lang/eval-okay-equal-function-direct-distinct-similar.exp

@@ -0,0 +1 @@
+false

tests/functional/lang/eval-okay-equal-function-direct-distinct-similar.nix

@@ -0,0 +1,3 @@
+# Direct comparison of distinct but not identical functions returns false
+# See https://nix.dev/manual/nix/latest/language/operators#equality
+(x: x) == (x: x)

tests/functional/lang/eval-okay-equal-function-direct-identical.exp

@@ -0,0 +1 @@
+false

tests/functional/lang/eval-okay-equal-function-direct-identical.nix

@@ -0,0 +1,6 @@
+# Direct comparison of identical function returns false
+# See https://nix.dev/manual/nix/latest/language/operators#equality
+let
+  f = x: x;
+in
+f == f

tests/functional/lang/eval-okay-equal-function-list-distinct-similar.exp

@@ -0,0 +1 @@
+false

tests/functional/lang/eval-okay-equal-function-list-distinct-similar.nix

@@ -0,0 +1,3 @@
+# Distinct but not identical functions in list compare as unequal
+# See https://nix.dev/manual/nix/latest/language/operators#equality
+[ (x: x) ] == [ (x: x) ]

tests/functional/lang/eval-okay-equal-function-list-identical.exp

@@ -0,0 +1 @@
+true

tests/functional/lang/eval-okay-equal-function-list-identical.nix

@@ -0,0 +1,6 @@
+# Function comparison in list uses value identity optimization
+# See https://nix.dev/manual/nix/latest/language/operators#value-identity-optimization
+let
+  f = x: x;
+in
+[ f ] == [ f ]