chore(core): add utf-8 encoding to Path read_text/write_text

[cbornet]

Summary

This PR standardizes all text file I/O to use UTF-8. This eliminates OS-specific defaults (e.g. Windows cp1252) and ensures consistent, Unicode-safe behavior across platforms.

Breaking changes

Users on systems with a default encoding which is not utf-8 may see decoding errors from the following code paths:

• langchain_core.vectorstores.in_memroy.InMemoryVectorStore.load
• langchain_core.prompts.loading.load_prompt
• from_template in AIMessagePromptTemplate, HumanMessagePromptTemplate, SystemMessagePromptTemplate

Migration

Change the encoding of files that are encoded with a non utf-8 encoding to utf-8.

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
langchain	Ready	Preview	Comment	Sep 2, 2025 9:45am

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #32784 will not alter performance

_{Comparing cbornet:plw1514 (d71acf9) with wip-v1.0 (25d5db8)}

⚠️ 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.

Summary

✅ 13 untouched benchmarks

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #32784 will not alter performance

_{Comparing cbornet:plw1514 (d71acf9) with wip-v1.0 (25d5db8)}

Summary

✅ 14 untouched benchmarks

[mdrxy]

https://docs.astral.sh/ruff/rules/unspecified-encoding/

Should ensure consistency

Also, windows doesn't use utf-8 by default so we may need to investigate remediation for those users

[cbornet]

NB: currently missing encoding in Path.read/write is not detected by ruff rule PLW1514 but it will be in the future (already detected with --preview flag)