Reject self-referential custom syntax includes

[jackwalkerlabs]

Summary

• reject custom include: scope: references that resolve back to the same custom syntax before syntect links them
• prevent cache files that can trigger unbounded highlighting recursion from being written
• cover both the recursive case and a valid include from another scope

Fixes #3878.

Testing

• cargo fmt -- --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked

AI assistance was used for implementation and test scaffolding; the final diff and validation output were inspected locally.