Pin chainrules merge revision

[shinaoka]

Summary

• Update the chainrules git dependency to the squash-merged main commit from Add fallible AD rule hooks chainrules-rs#13.
• Avoid depending on the deleted feature branch commit after auto-merge.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo nextest run --release --workspace --no-fail-fast
• cargo test --doc --release --workspace
• cargo llvm-cov nextest --workspace --release --json --output-path coverage.json
• python3 scripts/check-coverage.py coverage.json
• cargo doc --workspace --no-deps
• python3 scripts/check-docs-site.py

Implemented with OpenAI Codex.