fix: preserve conditional allOf branches

[FanouZeng-TT]

Description

merge_all_of_to_node flattens each allOf branch into the parent schema. For
generic keys it keeps only the first occurrence:

for k, v in item.items():
    if k not in reserved_keys and k not in node:
        node[k] = v

That loses conditional constraints when multiple branches each carry an
if/then pair. shopping/types/total.json, for example, has one branch that
requires discount amounts to be negative and another that requires positive
total types to be non-negative. After preprocessing, only the first if and
then survived.

Fix: keep conditional allOf branches intact instead of flattening their
if/then/else keys into the parent. A regression test supplies two distinct
conditional branches and verifies both remain after preprocessing.

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.
• 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)

• python -m unittest discover -s tests -p 'test_*.py' — 58 tests passed
• pre-commit run --all-files — passed
• ./generate_models.sh 2026-04-08 — completed; no generated model changes

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