fix: skip ref checks in instance data

[FanouZeng-TT]

Description

check_refs currently walks every object and array value in a schema. default, const, examples, and enum hold JSON instance data, so a business field named $ref in one of those values was treated as a schema reference and reported as E002.

Fix: skip those instance-data keyword values during reference traversal while preserving checks at real schema locations. The regression test covers all four keyword values and the existing broken-reference tests continue to cover real $ref values.

Category (Required)

• Core Protocol: Changes to core protocol specifications, breaking changes, or extensions. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance documentation or contribution guidelines. (Requires Governance Council approval)
• Capability: Changes to a capability specification. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: CI, build, or developer tooling changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependencies or 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): Community health files. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide.
• 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)

• cargo test --manifest-path /Users/bytedance/All/UCP/PR记录/worktrees/73-ucp-schema/Cargo.toml --all-targets
• cargo clippy --manifest-path /Users/bytedance/All/UCP/PR记录/worktrees/73-ucp-schema/Cargo.toml --all-targets -- -D warnings
• cargo fmt --manifest-path /Users/bytedance/All/UCP/PR记录/worktrees/73-ucp-schema/Cargo.toml -- --check
• uvx pre-commit run --all-files --show-diff-on-failure
• git diff --check