`_is_pk_set()` is incorrectly used to check for unsaved models.

[Clifford Gama]

Since models defined with custom primary keys can have pk set before being saved to the database, this means they bypass this as if they were saved. Here is an example:

tests/one_to_one/tests.py

@@ -170 +170 @@
         with self.assertRaises(Restaurant.DoesNotExist):
             place.restaurant
 
+    def test_unsaved_object_with_supplied_primary_key(self):
+        link1 = Place.objects.create(name="Ushaka", address="Ethekwini")
+        link2 = ManualPrimaryKey(primary_key="YTREWQ", name="moonwalk")
+        msg = (
+            "save() prohibited to prevent data loss due to unsaved related object "
+            "'link2'."
+        )
+        with self.assertRaisesMessage(ValueError, msg):
+            MultiModel.objects.create(link1=link1, link2=link2, name="Unsavable")
+
     def test_reverse_relationship_cache_cascade(self):
         """
         Regression test for #9023: accessing the reverse relationship shouldn't

which results in

FE
======================================================================
ERROR: test_unsaved_object_with_supplied_primary_key (one_to_one.tests.OneToOneTests.test_unsaved_object_with_supplied_primary_key)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/clifford/code/django/django/db/backends/sqlite3/base.py", line 304, in check_constraints
    raise IntegrityError(
django.db.utils.IntegrityError: The row in table 'one_to_one_multimodel' with primary key '1' has an invalid foreign key: one_to_one_multimodel.link2_id contains a value 'YTREWQ' that does not have a corresponding value in one_to_one_manualprimarykey.primary_key.

======================================================================
FAIL: test_unsaved_object_with_supplied_primary_key (one_to_one.tests.OneToOneTests.test_unsaved_object_with_supplied_primary_key)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/clifford/code/django/tests/one_to_one/tests.py", line 180, in test_unsaved_object_with_supplied_primary_key
    with self.assertRaisesMessage(ValueError, msg):
  File "/usr/lib/python3.12/contextlib.py", line 144, in __exit__
    next(self.gen)
AssertionError: ValueError not raised

----------------------------------------------------------------------
Ran 1 test in 0.004s

FAILED (failures=1, errors=1)

And here is the culprit ​code:
This is done in a few other places. My proposal is to replace such uses cases of _is_pk_set with instance._state.added checks.