Python: Read and write chat history files as UTF-8

[Kumaresan-AI-Engineer]

Motivation and Context

ChatHistory.store_chat_history_to_file and load_chat_history_from_file were opening files in text mode without specifying an encoding. This meant Python used the system's default locale encoding, which is typically cp1252 on Windows.

As a result, saving chat history containing non-ASCII characters (such as Chinese characters, accented characters, or emojis) could raise a UnicodeEncodeError. Similarly, chat history files written as UTF-8 could fail to load correctly on Windows.

This change makes both methods explicitly use UTF-8, which is also consistent with how other parts of the package handle text files, such as KernelPlugin.from_directory and KernelFunctionFromPrompt.from_directory.

Description
Explicitly use encoding="utf-8" when saving chat history.
Explicitly use encoding="utf-8" when loading chat history.
Added a regression test that saves and loads chat history containing Chinese, accented, and emoji characters.

The regression test ensures that chat history can be safely round-tripped across platforms, including Windows.

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[Copilot]

🟢 Approval recommended

The UTF-8 changes are covered by regression testing, with no unresolved review comments.

Pull request overview

Updates chat history file I/O to use UTF-8 consistently across platforms.

Changes:

• Uses UTF-8 when reading and writing chat history files.
• Adds regression coverage for non-ASCII content.

File summaries

File	Summary
python/tests/unit/contents/test_chat_history.py	Tests non-ASCII chat history round-tripping.
python/semantic_kernel/contents/chat_history.py	Applies UTF-8 encoding to file reads and writes.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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