test: add mutation testing with mutmut

Author: zurfjereluhmie

.gitignore

@@ -17,6 +17,7 @@ wheels/
 .coverage
 .coverage.*
 htmlcov/
+mutants/
 
 # Linting / type-checking caches
 .ruff_cache/

Makefile

@@ -1,6 +1,6 @@
 .DEFAULT_GOAL := help
 
-.PHONY: help install install-dev install-demo sync lint format typecheck test test-integration coverage check fix pre-commit docs docs-serve clean
+.PHONY: help install install-dev install-demo sync lint format typecheck test test-integration coverage mutate check fix pre-commit docs docs-serve clean
 
 help: ## Show this help message
 	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) \
@@ -36,6 +36,9 @@ test-integration: ## Run integration tests (requires Phoenix)
 coverage: ## Run pytest with coverage report
 	uv run pytest tests/ -q --cov=evalwire --cov-report=term-missing --cov-fail-under=85
 
+mutate: ## Run mutation testing with mutmut
+	uv run mutmut run
+
 check: lint format typecheck test ## Run all checks (lint, format, typecheck, test)
 
 fix: ## Auto-fix lint and format issues

pyproject.toml

@@ -69,12 +69,19 @@ markers = [
 ]
 addopts = "-m 'not integration'"
 
+[tool.mutmut]
+paths_to_mutate = ["src/evalwire/"]
+pytest_add_cli_args_test_selection = ["tests/"]
+pytest_add_cli_args = ["--ignore=tests/test_evaluators_property.py", "--ignore=tests/integration"]
+do_not_mutate = ["*__init__.py", "*cli.py"]
+
 [dependency-groups]
 dev = [
     "hypothesis>=6.152.1",
     "mkdocs>=1.6.1",
     "mkdocs-material>=9.0",
     "mkdocstrings[python]>=0.25",
+    "mutmut>=3.5",
     "pre-commit>=4.5.1",
     "pytest>=8.0",
     "pytest-asyncio>=0.23",