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/views.py

@@ -576,7 +576,7 @@ def bulk_pantry_add(request):
         name = item_data.get("ingredient_name", "").strip()
         qty_raw = item_data.get("quantity")
         quantity = int(qty_raw) if qty_raw not in (None, "") else None
-        unit = item_data.get("unit", "items")
+        unit = item_data.get("unit") or "items"  # tolerate null/empty
         location = item_data.get("location", "fridge")
         expiry_days = item_data.get("expiry_days")  # optional override