fix(openai): rely on langchain-core for setting chunk_position

[ccurme]

Better to rely on core for this (core emits a final chunk with chunk_position="last" if it's missing)

• When stream_usage is enabled, we are currently setting chunk_position="last" on the wrong chunk
• Behavior around this may vary with child classes of BaseChatOpenAI

[codspeed-hq]

CodSpeed Performance Report

Merging #34404 will not alter performance

_{Comparing cc/fix_openai_chunk_position (5ed30b7) with master (71778cb)}

Summary

✅ 8 untouched
⏩ 26 skipped¹

Footnotes

1. 26 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. ↩