[Feat]: implement the Cart capability in the Python sample server

[vishkaty]

Is your feature request related to a problem?

Cart (dev.ucp.shopping.cart) has been a released spec capability since the 2026-04-08 release (cart.md + cart-rest.md + shopping/cart.json), but the reference Python server doesn't implement it: POST /carts returns 404 and the capability isn't declared in the discovery profile (verified against current main).

That leaves the ecosystem without a reference for a released capability:

• implementers adding carts have no working example to model,
• the official conformance suite can't grow cart coverage (nothing to run it against),
• agent developers can't exercise cart flows end-to-end.

Describe the solution you'd like

Cart support in the Python sample mirroring the checkout implementation's structure: /carts lifecycle routes (create/get/update/cancel), cart-vs-checkout semantics per cart.md, capability + service declaration in the profile, and integration tests in the same style as the existing checkout tests.

Offer

I'd like to build this (my previous server contributions: #117, #128, #132/#120/#122 pending). Scoping questions before I start, so it lands the way you want:

1. Is cart intentionally deferred in the samples roadmap, or just not picked up yet?
2. Any preference on storage (reuse the transactions DB pattern?) and on whether cart→checkout conversion should be included in the first PR or staged?

If the answer is "planned internally," no problem — happy to redirect the effort.