fix: detect nullable allOf type conflicts

[FanouZeng-TT]

Description

validate_allof_types only compared string-form type keywords. An array-form type such as ["string", "null"] was skipped, so combining it with "number" in another allOf branch passed resolution even though no JSON instance can satisfy both branches.

Fix: normalize string and array forms into accepted JSON instance-type sets and intersect them across matching properties. Empty intersections keep returning the existing TypeConflict; overlapping nullable types and the JSON Schema number/integer relationship remain valid.

The regression test covers all three boundaries:

• ["string", "null"] with "number" reports a conflict
• ["string", "null"] with "string" resolves
• "number" with "integer" resolves

Category (Required)

• Core Protocol: Changes to core protocol specifications. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance or contributing processes. (Requires Governance Council approval)
• Capability: New or updated capabilities. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: Build, CI, or deployment changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependencies and repository maintenance. (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. (Requires Maintainer approval)
• Community Health (.github): Organization-wide community files. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide and Code of Conduct.
• 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 (not applicable).

Screenshots / Logs (if applicable)

N/A — verified with cargo test --all-targets, cargo clippy --all-targets -- -D warnings, cargo fmt --check, and all pre-commit hooks.