feat: add tax_included to the price type

[YanisMtcr]

Context: #256 (LATAM pricing) raised the same class of issue. This scopes it to the catalog price marker.

---

tax_included is a new optional field on the price type: a boolean stating whether the amount includes tax (VAT, GST, etc.). A catalog price already carries amount and currency; today it never says whether tax is in it, so an agent has to infer that from the buyer's country. This makes the price self-describing, mirroring how currency is already explicit, rather than adding new machinery.

What it looks like

A tax-inclusive EU catalog price:

{ "amount": 12000, "currency": "EUR", "tax_included": true }

A US catalog price omits it (or sets false); tax is added downstream. The itemized tax breakdown stays in checkout totals; this field only disambiguates the displayed price.

Optional and backward-compatible: when absent, behavior is unchanged; agents that ignore it fall back to today's inference, agents that read it drop the guesswork.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)

Checklist

• I have followed the Contributing Guide (Conventional Commits title, no breaking changes).
• I have updated the documentation.
• cspell and markdownlint pass locally. ucp-schema and super-linter not run locally (no cargo/docker); relying on CI.
• I have added tests that prove my fix is effective or that my feature works.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.