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-operator-not-function.err.exp

@@ -0,0 +1,10 @@
+error:
+       … while calling the 'genericClosure' builtin
+         at /pwd/lang/eval-fail-genericClosure-operator-not-function.nix:1:1:
+            1| builtins.genericClosure {
+             | ^
+            2|   startSet = [ { key = 1; } ];
+
+       … while evaluating the 'operator' attribute passed as argument to builtins.genericClosure
+
+       error: expected a function but found a string: "not a function"