fix: add encoding='utf-8' to file operations in task_centric_memory

[Sean-Kenneth-Doherty]

Why are these changes needed?

Follow-up to issue #5566 and PR #6094 which fixed encoding='utf-8' in playwright_controller.py.

The same issue exists in the task_centric_memory module where file operations don't specify explicit encoding. This causes UnicodeDecodeError on non-English Windows systems (e.g., cp950 for Traditional Chinese, cp936 for Simplified Chinese, cp932 for Japanese).

Files fixed:

• chat_completion_client_recorder.py: session file read/write operations (JSON that may contain non-ASCII characters)
• page_logger.py: hash file writes, call tree HTML, and page HTML writes (HTML content with potential non-ASCII text)

Without explicit encoding='utf-8', Python uses the system default encoding which varies by locale and can fail to handle UTF-8 content.

Related issue number

Related to #5566

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

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