Validate order products against catalog

[fuzziecoder]

Title

fix: enforce catalog integrity for order item creation
 
Body
Order items currently accept arbitrary name and unitPrice. Validate productId against catalog and compute trusted values server-side.
 
Acceptance Criteria

• Unknown productId is rejected.
• Backend derives item name/price from catalog.
• Client-provided price cannot override catalog value.

[dharapandya85]

Please assign me this issue.

[Shiwani-Nanaware]

I want to work on this issue under Apertre 3.0
Please assign to me and Add appropriate Apertre label.

[dharapandya85]

working on this

[fuzziecoder]

Sorry @Shiwani-Nanaware as per the guidelines the assigning will be in the basis of fcfs .so,you can contribute on other issues if you're interested in working with Any other issue let me know I'll assign it to you.