fix: skip annotation checks in instance data

[FanouZeng-TT]

Description

check_annotations currently walks every object and array value in a schema. JSON Schema keywords such as default, const, examples, and enum contain instance data rather than child schemas, so payload fields named ucp_request or ucp_response inside those values can be misreported as invalid UCP annotations.

This change skips those instance-data keywords during annotation traversal while preserving validation for real schema locations. It also adds a regression test covering annotation-like field names inside default, const, examples, and enum.

Category (Required)

• Fix
• Test
• Docs
• Chore
• CI

Related Issues

None.

Checklist

• I have read the contribution guidelines.
• I have added or updated tests as appropriate.
• I have run the relevant checks locally.

Screenshots / Logs (if applicable)

• cargo test --manifest-path /Users/bytedance/All/UCP/PR记录/worktrees/71-ucp-schema/Cargo.toml --all-targets
• cargo clippy --manifest-path /Users/bytedance/All/UCP/PR记录/worktrees/71-ucp-schema/Cargo.toml --all-targets -- -D warnings
• cargo fmt --manifest-path /Users/bytedance/All/UCP/PR记录/worktrees/71-ucp-schema/Cargo.toml -- --check
• uvx pre-commit run --all-files --show-diff-on-failure
• git diff --check

[igrigorik]

Thanks @FanouZeng-TT. #66 landed this guard centrally — INSTANCE_DATA_KEYWORDS plus a shared traversal in loader.rs — and wired it into the annotation check, so ucp_* keys inside const/enum/default/examples are treated as payload rather than schema.

To be clear about scope: only the annotation check is covered. The $ref and examples variants still reproduce on main, which is why #63 and #64 stay open.

Closing this one as fixed.