Fix formatting files with unicode

[mshafer-NI]

Adds a test for and fixes #168

[github-actions]

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[Copilot]

Pull Request Overview

This PR fixes issue #168 by ensuring proper Unicode handling when reading and writing Python files during formatting and linting operations.

Key Changes:

• Added explicit UTF-8 encoding to all file read/write operations across the codebase
• Replaced read_text()/write_text() with shutil.copyfile() in the diff generation path for better encoding preservation
• Added a test fixture that forces ASCII encoding by default to catch any missing encoding specifications
• Added comprehensive test cases with Unicode characters (©, π, Gerät, デバイス, 장치, 设备, etc.) to verify the fix

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_cli/test_format.py	Added explicit UTF-8 encoding to test file reading to properly handle Unicode content
ni_python_styleguide/_utils/code_analysis.py	Added UTF-8 encoding to file reading in import region analysis
ni_python_styleguide/_fix.py	Added DEFAULT_ENCODING to multiple read/write operations and replaced read_text/write_text with shutil.copyfile for better encoding preservation
tests/conftest.py	Added autouse test fixture to force ASCII encoding by default, catching any missing encoding specifications
pyproject.toml	Removed unnecessary blank line for code cleanup
tests/test_cli/format_test_cases__snapshots/file_with_unicode/*	Added new test case files containing various Unicode characters to validate the fix
tests/test_cli/fix_test_cases__snapshots/unicode_example/*	Added test case files for Unicode handling in the fix command
tests/test_cli/acknowledge_existing_errors_test_cases__snapshots/unicode_in_files/*	Extended existing test case with additional Unicode data

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