Expand PyCLIF pickle compatibility test coverage (NO production code changes).

These test were added while working on google/pybind11clif#30099.

Before google/pybind11clif#30099, the `SimpleCallable` pickle tests were failing with PyCLIF-pybind11.

PiperOrigin-RevId: 608694227

Author: rwgk

clif/testing/pickle_compatibility.h

@@ -22,6 +22,13 @@
 
 namespace clif_testing {
 
+inline int SimpleCallable() { return 234; }
+
+struct SimpleStruct {
+  int value = -987;
+  int SimpleMethod() { return value; }
+};
+
 class StoreTwoUsingPostproc {
  public:
   StoreTwoUsingPostproc(int v0, int v1) : values{v0, v1} {}

clif/testing/python/pickle_compatibility.clif

@@ -16,6 +16,11 @@ from clif.testing.python.pickle_compatibility_helper import ReduceStoreTwo
 
 from "clif/testing/pickle_compatibility.h":
   namespace `clif_testing`:
+    def SimpleCallable() -> int
+
+    class SimpleStruct:
+      def SimpleMethod(self) -> int
+
     class StoreTwoUsingPostproc:
       def __init__(self, v0: int, v1: int)
       def Get(self, i: int) -> int

clif/testing/python/pickle_compatibility_test.py

@@ -40,6 +40,29 @@
 
 class PickleCompatibilityTest(unittest.TestCase):
 
+  def testAssumptions(self):
+    self.assertGreaterEqual(pickle.HIGHEST_PROTOCOL, 0)
+
+  @parameterized.parameterized.expand(range(pickle.HIGHEST_PROTOCOL + 1))
+  def testSimpleCallable(self, protocol):
+    self.assertEqual(pickle_compatibility.SimpleCallable(), 234)
+    serialized = pickle.dumps(
+        pickle_compatibility.SimpleCallable, protocol=protocol
+    )
+    self.assertIn(b'clif.testing.python._pickle_compatibility', serialized)
+    self.assertIn(b'SimpleCallable', serialized)
+    deserialized = pickle.loads(serialized)
+    self.assertEqual(deserialized(), 234)
+    self.assertIs(deserialized, pickle_compatibility.SimpleCallable)
+
+  def testSimpleMethod(self):
+    obj = pickle_compatibility.SimpleStruct()
+    self.assertEqual(obj.SimpleMethod(), -987)
+    with self.assertRaisesRegex(
+        TypeError, 'missing __getinitargs__ and/or __getstate__'
+    ):
+      pickle.dumps(obj.SimpleMethod)
+
   @parameterized.parameterized.expand(zip(EMPTY_TYPES))
   def testUnpicklable(self, empty_type):
     obj = empty_type()