Fix Optional[Shape] unwrapping and add comprehensive shapes stress tests

_extract_shape_class now handles `Shape | None` (Union types) by checking
isinstance(hint, types.UnionType) and iterating args for Shape subclasses.
This fixes nullable FK detection — any `editor: AuthorShape | None` field
is now correctly recognized as a nested shape.

48 stress tests covering:
- 5-level deep nesting (Publisher → Author → Book → Chapter → Section)
- Two FKs to same model (author + editor)
- Slug PK (Tag), UUID PK (Section)
- M2M relationships (Book.tags)
- Nullable FKs returning None
- Empty strings, zero integers, false booleans (truthiness traps)
- 100-record smoke test
- Query efficiency (assertNumQueries)
- All diff operations with deep nesting

Known gap documented: self-referential forward refs (CategoryShape)
crash get_type_hints() at __init_subclass__ time. Needs deferred resolution.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

django/tests/models.py

@@ -42,33 +42,91 @@ class EmailUser(AbstractBaseUser, PermissionsMixin):
 
 # ─── Shape test models ──────────────────────────────────────────────────────
 
+import uuid
 
-class Author(models.Model):
-    name = models.CharField(max_length=100)
-    bio = models.TextField(blank=True, default="")
+
+class TimestampMixin(models.Model):
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        abstract = True
+
+
+class Publisher(TimestampMixin):
+    name = models.CharField(max_length=200)
+    country = models.CharField(max_length=100, default="")
 
     class Meta:
         app_label = "tests"
 
-    def __str__(self):
-        return self.name
 
-
-class Book(models.Model):
-    title = models.CharField(max_length=200)
-    pages = models.IntegerField(default=0)
-    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name="books")
+class Author(TimestampMixin):
+    name = models.CharField(max_length=200)
+    bio = models.TextField(default="")
+    publisher = models.ForeignKey(
+        Publisher, on_delete=models.CASCADE, related_name="authors"
+    )
+    mentor = models.ForeignKey(
+        "self", on_delete=models.SET_NULL, null=True, blank=True, related_name="mentees"
+    )
 
     class Meta:
         app_label = "tests"
 
-    def __str__(self):
-        return self.title
-
 
 class Tag(models.Model):
-    name = models.CharField(max_length=50)
-    books = models.ManyToManyField(Book, related_name="tags", blank=True)
+    slug = models.SlugField(primary_key=True, max_length=100)
+    label = models.CharField(max_length=100)
+
+    class Meta:
+        app_label = "tests"
+
+
+class Book(TimestampMixin):
+    title = models.CharField(max_length=300)
+    isbn = models.CharField(max_length=13, unique=True)
+    page_count = models.IntegerField(default=0)
+    is_published = models.BooleanField(default=False)
+    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name="books")
+    editor = models.ForeignKey(
+        Author, on_delete=models.SET_NULL, null=True, blank=True, related_name="edited_books",
+    )
+    tags = models.ManyToManyField(Tag, blank=True, related_name="books")
+
+    class Meta:
+        app_label = "tests"
+
+
+class Chapter(TimestampMixin):
+    book = models.ForeignKey(Book, on_delete=models.CASCADE, related_name="chapters")
+    number = models.IntegerField()
+    title = models.CharField(max_length=300)
+    word_count = models.IntegerField(default=0)
+
+    class Meta:
+        app_label = "tests"
+        ordering = ["number"]
+        unique_together = [("book", "number")]
+
+
+class Section(models.Model):
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    chapter = models.ForeignKey(Chapter, on_delete=models.CASCADE, related_name="sections")
+    heading = models.CharField(max_length=300)
+    body = models.TextField(default="")
+    position = models.IntegerField(default=0)
+
+    class Meta:
+        app_label = "tests"
+        ordering = ["position"]
+
+
+class Category(models.Model):
+    name = models.CharField(max_length=200)
+    parent = models.ForeignKey(
+        "self", on_delete=models.CASCADE, null=True, blank=True, related_name="children"
+    )
 
     class Meta:
         app_label = "tests"