fix: Fix inconsistent schema on documentation examples & discounts extension schema

[jingyli]

Description

This PR aims to address some minor schema inconsistencies that are present in the current UCP spec:

1. In discount.md: Update all the examples to follow the correct UCP schema - item.id is the only field populated in requests, quantity pulled out to the line_item level, etc.
2. In checkout-rest.md and checkout-mcp.md: Remove unsupported available_quantity field and add in other required fields like item in business error example.
3. Fix incorrect usage of readOnly annotation in discount.json.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Documentation update

---

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[cusell-google]

Some automated doc checks were skipped, could you make sure they run before merging?

[kmcduffie]

@jingyli Is it worth modeling this as the price.json obj that is both amount and currency?
https://github.com/Universal-Commerce-Protocol/ucp/blob/6b61c3f7dab1a7825dc6c8a7c8c433d426db21d5/source/schemas/shopping/types/price.json

[jingyli]

My thinking is that the current model is more explicit: we already have currency at the checkout object level - https://github.com/Universal-Commerce-Protocol/ucp/blob/main/source/schemas/shopping/checkout.json#L74 - and our expectation is that all amounts in the checkout object should follow that currency field.

[jingyli]

@cusell-google Thanks for the review! The skipped checks are expected - those checks should only be executed as part of the release workflow.

[raginpirate]

Technically this is a breaking change, because the readOnly was not our expected control over the output schemas and validation here. But we should be fine accepting this given our belief that consumers of the spec will not be impacted and would have interpreted this the same way.

[mmohades]

Thanks for catching this inconsistency in the docs!

[igrigorik]

👍

[westeezy]

lgtm ty.