fix: update_checkout_session to handle all optional args correctly.

.cspell/custom-words.txt

@@ -38,6 +38,8 @@ Visa
 Wayfair
 Worldpay
 Zalando
+absl
+absltest
 adyen
 agentic
 atok
@@ -52,6 +54,7 @@ fontawesome
 fpan
 fulfillable
 gpay
+healthz
 ingestions
 inlinehilite
 linenums
@@ -75,4 +78,4 @@ shopify
 superfences
 vulnz
 yaml
-yml
+yml

integration_test_utils.py

@@ -778,7 +778,25 @@ def update_checkout_session(
 
     """
     # Default to existing values if not provided
-    currency = currency if currency is not None else checkout_obj.currency
+    if currency is None:
+      currency = checkout_obj.currency
+
+    def to_json(obj):
+      if obj is not None and hasattr(obj, "model_dump"):
+        return obj.model_dump(mode="json", exclude_none=True)
+      return obj
+
+    if buyer is None:
+      buyer = to_json(getattr(checkout_obj, "buyer", None))
+
+    if fulfillment is None:
+      fulfillment = to_json(getattr(checkout_obj, "fulfillment", None))
+
+    if discounts is None:
+      discounts = to_json(getattr(checkout_obj, "discounts", None))
+
+    if platform is None:
+      platform = to_json(getattr(checkout_obj, "platform", None))
 
     # Construct Line Items
     if line_items is None: