docs: fix discount fixture percentage keys

[FanouZeng-TT]

Description

The custom fixture example documents amount-based keys for percentage discount tests:

"expected_discount_reduction": 1.0,
"expected_discount_reduction_2": 1.8

DynamicFixtureContext does not read either key. business_logic_test.py uses get_expected_discount_percentage() and get_expected_discount_percentage_2(), which look for expected_discount_percentage and expected_discount_percentage_2; following the README therefore falls back to 0.0 and makes the expected discount amount zero.

The bundled flower-shop fixture already uses the percentage keys with values 10.0 and 20.0.

Fix: align the README example and field descriptions with the percentage keys consumed by the test suite.

Category (Required)

• 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

N/A

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. (The README fixture is parsed through the production fixture reader.)
• 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. (Not applicable; no schema or model changes.)

Screenshots / Logs (if applicable)

README fixture through DynamicFixtureContext: 10.0%, 20.0%
ruff check / ruff format --check: passed
pre-commit: 12 hooks passed