POST /Booking returns 500 Internal Server Error instead of helpful 400 respons #1547

When API clients attempt to create Booking or BookingSupplement entities directly via POST /processor/Booking, they receive a 500 Internal Server Error that doesn't explain what went wrong or how to fix it.

Steps to Reproduce

curl -X POST "http://localhost:4004/processor/Booking" \
  -u "amy:" \
  -H "Content-Type: application/json" \
  -d '{
    "BookingUUID": "01234567-89ab-cdef-0123-456789abcdef",
    "BookingDate": "2017-04-13",
    "ConnectionID": "",
    "FlightDate": "2017-04-13",
    "FlightPrice": 0,
    "CurrencyCode_code": "",
    "BookingStatus_code": "",
    "to_BookSupplement": [],
    "to_Carrier_AirlineID": "",
    "to_Customer_CustomerID": "",
    "to_Travel_TravelUUID": "01234567-89ab-cdef-0123-456789abcdef"
  }'

Expected Behavior

A 400 Bad Request with a helpful message explaining that Booking entities must be created via their parent Travel entity:

{
  "code": "400",
  "message": "Bookings must be created in the context of a Travel draft. Use POST /processor/Travel(TravelUUID=<uuid>,IsActiveEntity=false)/to_Booking instead."
}

Actual Behavior

{
  "error": {
    "message": "Internal Server Error",
    "code": "500"
  }
}

Root Cause

1. Booking and BookingSupplement are composition entities that can only be created via their parent (Travel → Booking → BookingSupplement).
2. The error occurs at the database layer and bubbles up as a generic 500 Internal Server Error.
3. No validation exists to catch this earlier and provide a helpful, actionable error message.

Proposed Solution

Add request validation handlers in srv/travel-service.ts:

• A before handler to reject direct POST /Booking requests before they reach the database.
• An on('error') handler as a fallback to catch any remaining database-level errors and convert them from 500 Internal Server Error to 400 Bad Request with actionable guidance.

Additional Issues Found

Potential null pointer crashes in existing handlers in srv/travel-service.ts:

• update_totalsGreen() crashes when the Travel draft cannot be found.
• deductDiscount() accesses travel.ID when travel is null.
• Date validation is missing null checks before comparing dates.

Environment

• @sap/cds: 10.x
• Node.js: 22.x