fix(openai): update file index key name

[ccurme]

Collision with the "index" key, originally reported here: https://forum.langchain.com/t/400-error-from-concatenated-annotation-fields-during-chunk-aggregation-openai-built-in-file-search/1767

[codspeed-hq]

CodSpeed Performance Report

Merging #33350 will not alter performance

_{Comparing cc/openai_file_search (52c5f8b) with master (a3e4f4c)}

Summary

✅ 6 untouched
⏩ 28 skipped¹

Footnotes

1. 28 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩