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>

mcp_server/server.py

@@ -73,15 +73,18 @@ def add_to_pantry(items: list[dict]) -> dict:
     """Add items the user bought. Each item is {name, location?, quantity?,
     unit?} — e.g. {"name": "pork mince", "location": "fridge"}. Existing items
     are restocked (set back to In) rather than duplicated."""
-    payload = [
-        {
+    payload = []
+    for it in items:
+        entry = {
             "ingredient_name": it.get("name") or it.get("ingredient_name"),
             "location": it.get("location", "fridge"),
-            "quantity": it.get("quantity"),
-            "unit": it.get("unit"),
         }
-        for it in items
-    ]
+        # Only forward optional fields when set, so the API applies its defaults.
+        if it.get("quantity") is not None:
+            entry["quantity"] = it["quantity"]
+        if it.get("unit"):
+            entry["unit"] = it["unit"]
+        payload.append(entry)
     try:
         return _api.bulk_add(payload)
     except FoodApiError as e: