Add unit tests

[XZman]

Add unit tests (pytest), coverage test, python language linting, and CI workflow.

[Copilot]

Pull request overview

This PR adds comprehensive unit testing infrastructure, including pytest configuration, code coverage tracking, linting with ruff and mypy, and CI/CD workflows via GitHub Actions. The changes include new test files for various components, test fixtures (JSON files), code refactoring to improve testability, and automation scripts.

Changes:

• Added unit tests for frontend operators, utilities, and configuration classes
• Configured pytest with coverage tracking and test discovery options
• Integrated ruff and mypy for code quality and type checking
• Set up GitHub Actions workflows for automated testing and linting

Reviewed changes

Copilot reviewed 105 out of 119 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pyproject.toml	Added dev dependencies (pytest-cov, ruff, mypy) and tool configurations
neusim/npusim/frontend/tests/test_operator.py	New unit tests for Operator classes and CSV serialization
neusim/npusim/frontend/tests/test_frontend_util.py	New tests for frontend utility functions
neusim/npusim/frontend/tests/assets/raw/*	Test fixture JSON files for various models
neusim/npusim/backend/util.py	Moved parse_input/output_tensor_shapes functions here
neusim/npusim/backend/tests/test_util.py	Deleted old test file
neusim/npusim/backend/npusim_lib.py	Updated to use moved utility functions
neusim/configs/tests/*	New unit tests for configuration classes
neusim/configs/models/LLMConfig.py	Fixed default value handling bug
lint.sh	New shell script for running linting checks
.github/workflows/*.yml	GitHub Actions workflows for CI/CD
README.md	Added CI status badges

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