Pin UTF-8 encoding when reading user-provided config files

[dltsum]

What

Two user-facing config reads fall back to the locale encoding:

• fetch_original_config reading the user-supplied original config file (src/diffusers/loaders/single_file_utils.py) — the entry point used by from_single_file when a local YAML config is passed
• the fp16_safetensors conversion command reading model_index.json (src/diffusers/commands/fp16_safetensors.py)

Why

On Windows with a non-UTF-8 locale (e.g. cp936/GBK), any non-ASCII content in these files crashes with UnicodeDecodeError. Original config files are user-authored YAML where non-ASCII comments or values are realistic.

Reproduction (Windows, cp936 locale)

from diffusers.loaders.single_file_utils import fetch_original_config

open("cfg.yaml", "w", encoding="utf-8").write("# 汉 注释\nmodel:\n  desc: 汉 模型\n")
fetch_original_config("cfg.yaml")
# UnicodeDecodeError: 'gbk' codec can't decode byte 0x89 in position 4: illegal multibyte sequence

Verified red before the fix; after the fix the file is read and parsed correctly (non-ASCII values preserved).

🤖 Generated with Claude Code

[github-actions]

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