fix: add explicit encoding='utf-8' to text file operations

[GopalGB]

Summary

• Adds encoding="utf-8" to all open() calls for text files across magentic-one-cli, autogen-studio, and agbench packages
• Prevents UnicodeDecodeError on non-English Windows systems where Python defaults to the system locale encoding (e.g., cp1252, gbk)
• 7 files updated, 24 open() calls fixed

Files Changed

• magentic-one-cli/src/magentic_one_cli/_m1.py — config file reads
• autogen-studio/autogenstudio/database/schema_manager.py — Alembic config/template reads and writes
• agbench/src/agbench/run_cmd.py — scenario files, env configs, run scripts
• agbench/src/agbench/tabulate_cmd.py — console log reads
• agbench/src/agbench/remove_missing_cmd.py — console log reads
• agbench/src/agbench/linter/cli.py — log file reads
• agbench/src/agbench/linter/coders/oai_coder.py — cache file reads/writes

Test Plan

• All changes are mechanical: adding encoding="utf-8" parameter only
• No behavioral change on systems where UTF-8 is already the default
• Fixes crashes on Windows with non-English locale (e.g., Chinese, Japanese, Korean)

Fixes #5566

[GopalGB]

@microsoft-github-policy-service agree

[GopalGB]

Rebased onto latest microsoft/autogen:main (was 37 days BEHIND) and re-authored under GopalGB <67310594+GopalGB@users.noreply.github.com> for consistent attribution. CI should now run on a fresh state. Ready for review when maintainers have cycles.