Harden build_line_item_summary against chaos key-rename injection + add unit tests

[Copilot]

Recurring chaos injection renames unit_price → price in build_line_item_summary(), causing KeyError crashes on every POST /api/demo/simulate-load and POST /api/orders/{id}/process call.

Changes

apps/api/app/routes/orders.py

• Replace direct key access with .get() + safe defaults throughout build_line_item_summary() — eliminates KeyError regardless of field name mutation
• Emit logger.warning when unit_price or quantity are absent, making chaos injection immediately visible in Application Insights without crashing

# Before — crashes on chaos injection
total = item["unit_price"] * item["quantity"]

# After — degrades gracefully, warns in logs
if "unit_price" not in item:
    logger.warning("build_line_item_summary: missing 'unit_price' key in item %s; defaulting to 0.0", ...)
unit_price = item.get("unit_price", 0.0)
quantity   = item.get("quantity", 0)
total      = unit_price * quantity

apps/api/tests/test_orders.py (new)

Five unit tests covering: correct keys, chaos "price" key (no KeyError), missing quantity (no ZeroDivisionError), zero quantity, and missing sku/name fallbacks.