fix(core): update on_llm_new_token type

[mdrxy]

Supersedes #34727

Closes #30703

Related:

• on_llm_new_token receives list instead of string with Gemini 3 models langchain-google#1460
• fix(genai): Normalize streaming token format for Gemini models langchain-google#1501

[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/update-on_llm_new_token-type (ea18e7f) with master (2ff1d23)²}

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. ↩

2. No successful run was found on master (1ead03c) during the generation of this report, so 2ff1d23 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩