fix: validate examples through internal refs

[FanouZeng-TT]

Description

check_examples compiled each schema node in isolation. A node that inherited its constraints through an internal $ref such as #/$defs/positive therefore lost the root document needed to resolve that reference. Validator compilation failed and the best-effort path skipped the examples, allowing an invalid documented value to pass linting.

Fix: register the root schema as an in-memory resource and validate each examples array through a JSON Pointer to its enclosing schema node. Pointer tokens are escaped before building the reference, and unresolved external references keep the existing best-effort behavior.

The regression test uses a referenced positive-integer schema with examples [0, 2]: only 0 produces E008. The updated linter also passes all 104 JSON documents under the UCP source/ tree with no errors.

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, all pre-commit hooks, and a full lint of the UCP source/ tree.