fix: add explicit utf-8 encoding to file open calls

[IgnazioDS]

Summary

• Add encoding='utf-8' to open() calls across the codebase (15 files)
• The original fix for playwright_controller.py (line 67) was already merged in PR add utf encoding in websurfer read file #6094, but the issue remains open because similar missing encoding parameters exist throughout the codebase
• Prevents UnicodeDecodeError on non-English Windows systems (e.g., CP950 for Traditional Chinese)

Problem

On Windows systems with non-UTF-8 default encoding, open() without explicit encoding crashes with UnicodeDecodeError: 'cp950' codec can't decode byte. See #5566.

Changes

magentic-one-cli:

• _m1.py — config file reading

autogen-ext:

• docker_jupyter/_docker_jupyter.py — HTML file writing

autogen-studio:

• database/schema_manager.py — alembic config read/write (7 calls)
• web/auth/manager.py — YAML config reading
• lite/studio.py — env file writing
• cli.py — env file writing
• gallery/builder.py — JSON file writing

agbench:

• run_cmd.py — env/config file reading (4 calls)
• tabulate_cmd.py — console log reading (2 calls)
• remove_missing_cmd.py — console log reading
• linter/cli.py — log file reading
• linter/coders/oai_coder.py — cache file read/write
• benchmarks/process_logs.py — log/prompt file reading (8 calls)
• benchmarks/GAIA/Scripts/custom_tabulate.py — answer/log file reading

Other:

• fixup_generated_files.py — generated file read/write

Test plan

• No regression on Linux/macOS where UTF-8 is typically the default
• Files read/write correctly on Windows with non-UTF-8 locale
• All existing tests pass (encoding='utf-8' is a safe default for all text files in this repo)

Fixes #5566

[IgnazioDS]

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