tests: add error tests for builtins.genericClosure

Covers error conditions for:
- Invalid argument types (not an attrset)
- Missing required attributes (startSet, operator)
- Type mismatches (startSet/operator not correct type)
- Element validation (elements not attrsets, missing key attribute)
- Key comparison errors (incompatible types, uncomparable types)
- Operator return value validation (not a list)

tests/functional/lang/eval-fail-genericClosure-keys-uncomparable.nix

@@ -0,0 +1,7 @@
+builtins.genericClosure {
+  startSet = [
+    { key = { }; }
+    { key = { }; }
+  ];
+  operator = x: [ ];
+}