fix(api): SegmentType.is_valid() raises AssertionError for SegmentType.GROUP (#28249)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

api/core/variables/types.py

@@ -1,9 +1,12 @@
 from collections.abc import Mapping
 from enum import StrEnum
-from typing import Any, Optional
+from typing import TYPE_CHECKING, Any, Optional
 
 from core.file.models import File
 
+if TYPE_CHECKING:
+    pass
+
 
 class ArrayValidation(StrEnum):
     """Strategy for validating array elements.
@@ -155,6 +158,17 @@ class SegmentType(StrEnum):
             return isinstance(value, File)
         elif self == SegmentType.NONE:
             return value is None
+        elif self == SegmentType.GROUP:
+            from .segment_group import SegmentGroup
+            from .segments import Segment
+
+            if isinstance(value, SegmentGroup):
+                return all(isinstance(item, Segment) for item in value.value)
+
+            if isinstance(value, list):
+                return all(isinstance(item, Segment) for item in value)
+
+            return False
         else:
             raise AssertionError("this statement should be unreachable.")