Fix bulk-pantry-add crash on null unit (found in live MCP testing)

End-to-end testing (MCP client -> FastMCP -> Django) surfaced a 500:
adding an item with an explicit `unit: null` (as add_to_pantry sends
for items without a unit) hit a NOT NULL violation, because
`item_data.get("unit", "items")` returns None when the key is present.

- views.bulk_pantry_add: `item_data.get("unit") or "items"` — tolerate
  null/empty unit.
- mcp_server add_to_pantry: omit quantity/unit from the payload when
  unset, so the API applies its own defaults.
- test: bulk-add with unit/quantity = null returns 201 (25 pass).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

kitchen/tests.py

@@ -224,6 +224,19 @@ class BulkAddApiTests(_ApiTestCase):
         self.assertEqual(rows.count(), 1)
         self.assertEqual(rows.first().state, "in")
 
+    def test_tolerates_null_unit_and_quantity(self):
+        # An MCP client may send unit/quantity as null for unknown items.
+        r = self.client.post(
+            "/api/bulk-pantry-add/",
+            {"items": [{"ingredient_name": "pork mince", "location": "fridge",
+                        "unit": None, "quantity": None}]},
+            format="json",
+        )
+        self.assertEqual(r.status_code, 201)
+        self.assertTrue(
+            PantryItem.objects.filter(ingredient__name="pork mince", location="fridge").exists()
+        )
+
 
 class WhatCanICookApiTests(_ApiTestCase):
     def setUp(self):