fix: enforce utf-8 encoding globally to resolve #5566

[wasim-builds]

Closes #5566

Problem

UnicodeDecodeError / UnicodeEncodeError on non-English systems because file open calls omit encoding='utf-8'.

Fix

Enforce UTF-8 encoding on all open() calls across the codebase.

Scope

34 files updated with explicit encoding='utf-8'. Covers benchmarks, samples, autogen-ext, autogen-studio, and CLI packages.

Verification

All file I/O operations succeed on Windows (GBK/CP1252), Linux, and macOS regardless of system locale.

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