feat: Add Payment Terms with Well-Defined Schedules

[raginpirate]

Description

Introduces dev.ucp.shopping.payment_terms, an extension for expressing checkout-wide payment terms composed of immediate, deferred, or finite recurring payment schedules.

Businesses advertise complete terms through payment.available_terms[], including schedule timing and totals. Buyers select and fund a term directly through payment.instruments[].term_refs, which can target the whole term or individual schedules. This naturally composes with Split Payments—for example, using a gift card for an immediate deposit and a card for a deferred balance.

The resulting Order snapshots the complete accepted term under order.payment.terms[]. The model is designed to compose with the read-only Order payment instruments proposed in #584, preserving schedule assignments without carrying checkout-time credentials or handler details.

The structured schedule model gives platforms and businesses a shared way to determine:

• Exactly how much is due for each payment occurrence, empowering interesting SDK usage like Apple Pay's recurring line items.
• What terms should be bound into an AP2 mandate for ingestion as verifiable proof of a recurring term.

Category (Required)

Please select one or more categories that apply to this change.

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance 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)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps 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)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• 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.
• (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.

Screenshots / Logs (if applicable)