fix: add encoding='utf-8' to open() calls in autogen-ext

[ddarji1409]

Summary

Fixes UnicodeDecodeError on systems with non-UTF-8 default encodings (e.g. Windows with cp950, cp932, or other CJK codecs) by explicitly specifying encoding='utf-8' on all text-mode open() calls in the autogen-ext package.

Closes #5566

Files changed

• autogen_ext/code_executors/docker_jupyter/_docker_jupyter.py — _save_html() write
• autogen_ext/experimental/task_centric_memory/utils/chat_completion_client_recorder.py — session file read & write
• autogen_ext/experimental/task_centric_memory/utils/page_logger.py — hash, call-tree, and page HTML writes

Root cause

Python's open() defaults to the platform locale encoding (locale.getpreferredencoding()). On East Asian Windows systems this is often cp950 or cp932, which cannot decode the UTF-8 encoded content written by other parts of the codebase (e.g. page_script.js which already had the fix applied). Explicitly passing encoding='utf-8' makes behaviour consistent across all platforms.

Test plan

• Reproduce issue on a system with non-UTF-8 locale (e.g. PYTHONIOENCODING=cp950) and confirm the error is gone
• Run existing test suite: pytest python/packages/autogen-ext/tests/
• Verify no functional regression on default UTF-8 systems

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