fix: add encoding='utf-8' to all text-mode open() calls

[haroldfabla2-hue]

Fix for #5566

Problem

On systems with non-UTF-8 default encodings (e.g. cp950 on Traditional Chinese Windows, cp932 on Japanese Windows), open() calls without explicit encoding use the locale default. This causes UnicodeDecodeError when reading UTF-8 content like bundled JS files, config files, or session recordings.

Root Cause

The original bug report (#5566) identified playwright_controller.py — that specific call was already fixed in main. However, as the reporter noted: "there will be some similar issues in code base while using open function." This PR addresses all remaining text-mode open() calls.

Changes

Added encoding="utf-8" to 49 text-mode open() calls across 20 files in 4 packages:

Package	Files	Key fixes
magentic-one-cli	1	Config file reads (_m1.py) — the exact entry point from the original bug report
autogen-ext	4	chat_completion_client_recorder.py (session record/replay), page_logger.py (HTML logs), mcp_session_host_example.py, _docker_jupyter.py
autogen-studio	5	schema_manager.py, auth/manager.py, gallery/builder.py, cli.py, lite/studio.py
agbench	10	run_cmd.py, tabulate_cmd.py, linter/, benchmark scripts

Binary mode ("rb", "wb") calls were intentionally left unchanged since encoding does not apply.

Verification

# Confirm no text-mode open() without encoding remains
grep -rn "with open(" python/packages/ --include="*.py" | grep -v encoding | grep -v "aiofiles\|Image.open\|io\.\|BytesIO\|subprocess\|mock\|webbrowser" | grep -v "test\|__pycache__" | grep "\"r\"\|\"w\"\|\"rt\"\|\"wt\""
# Result: (empty)

Testing

• ruff check passes (formatting preserved)
• Full test suite (CI will verify)

Fixes #5566

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