test: add schema preprocessing coverage and CI

[ShuoRen-TT]

Description

preprocess_schemas.py controls schema normalization and request-variant generation, but the repository did not have an automated test suite for this pipeline. This PR adds 13 standard-library unit and integration-style tests covering:

• local and external reference normalization
• allOf flattening and entity-field inlining
• ucp_request operation inclusion rules
• object and array-root request variant generation
• transitive variant dependencies and metadata normalization
• the complete preprocessing pipeline using a temporary schema tree

It also adds a minimal GitHub Actions matrix for Python 3.10, 3.11, and 3.12, plus the corresponding local test command in the README. There are no production-code or generated-model changes.

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

Follow-up regression coverage for #52. This PR does not close an issue.

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 applicable local linting and formatting checks.
• I have added tests that prove the preprocessing behavior is effective.
• New and existing unit tests pass locally with my changes.
• (Not applicable) This PR does not change Core/Capability JSON schemas.
• (Not applicable) This PR does not change generated models.

Screenshots / Logs (if applicable)

• python3 -m unittest discover -s tests -p 'test_*.py' -v: 13 tests passed
• ruff check and ruff format --check: passed
• Prettier 3.6.2, Codespell, workflow YAML parsing, and git diff --check: passed

The repository-pinned mirrors-prettier@v4.0.0-alpha.8 pre-commit environment could not initialize locally because npm could not resolve tiny-levenshtein@^1.1.0. The applicable changed files were checked directly with stable Prettier 3.6.2 and passed.

[damaz91]

Thanks a lot @ShuoRen-TT for working on this. LGTM!