fix: add explicit UTF-8 encoding to file operations in playwright_controller

[Spectual]

Summary

Add explicit encoding="utf-8" parameter to open() calls in playwright_controller.py and related files in the ext-agent-surfer package.

Motivation

On non-English Windows systems, open() defaults to the system locale encoding (e.g., cp1252, gbk) rather than UTF-8. This causes UnicodeDecodeError when reading files containing non-ASCII characters.

Fixes #5566

Changes

• Added encoding="utf-8" to all text file open() calls in playwright_controller.py
• Also fixed similar issues in other files within the same package for consistency

Testing

• Verified syntax correctness of all modified files
• Changes are minimal and low-risk (only adding encoding parameter to existing open calls)

[Spectual]

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