Short title (summary): Fix memory leaks in yyjson benchmarks (missing yyjson_doc_free)

Description

• Some yyjson-based benchmarks did not call yyjson_doc_free, causing large memory leaks when running multiple tests (e.g., ./bench_ondemand without filters could exhaust 32 GB RAM).
• Fixed by ensuring every yyjson_doc created in the benchmarks is properly freed. After this change, Valgrind reports no leaks on affected benchmarks.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation / tests
• Other (please describe):

How to verify / test

• Run benchmarks with Valgrind: valgrind --leak-check=full --trace-children=yes build/benchmark/bench_ondemand
• No more “definitely lost” reports after the changes in this PR.