fix(config): load manim.cfg beside input file

[emerardd]

Closes #4963.

Summary

• Reload folder-wide configuration from the input file's directory before applying CLI arguments.
• Ignore a manim.cfg in the execution directory when rendering a scene elsewhere.
• Preserve explicit --config_file, stdin/Jupyter handling, and directory arguments whose config supplies input_file.
• Add regression coverage for scene-adjacent config lookup and directory-based input config.

Validation

• 8 focused configuration and CLI tests
• Ruff check and formatting
• Targeted mypy check for manim/_config/utils.py
• git diff --check