fix(core): preserve index and timestamp fields when merging

[Mason Daugherty (mdrxy)]

Porting langchain-ai/langchainjs#9781

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 13 untouched benchmarks
⏩ 21 skipped benchmarks¹

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_{Comparing mdrxy/temporal-id-values (5e87e7d) with master (0b99ca4)}

Footnotes

1. 21 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. ↩