refactor: Improve code quality, error handling, and test coverage

- Add file_lock context manager to eliminate duplicate locking patterns
- Add ValidationError, ConfigurationError, CertificateError exceptions
- Improve rollback logic in haproxy_add_servers (track successful ops only)
- Decompose haproxy_add_domain into smaller helper functions
- Consolidate certificate constants (CERTS_DIR, ACME_HOME) to config.py
- Enhance docstrings for internal functions and magic numbers
- Add pytest framework with 48 new tests (269 -> 317 total)
- Increase test coverage from 76% to 86%
  - servers.py: 58% -> 82%
  - certificates.py: 67% -> 86%
  - configuration.py: 69% -> 94%

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

haproxy_mcp/pyproject.toml

@@ -6,3 +6,20 @@ requires-python = ">=3.11"
 dependencies = [
     "mcp[cli]>=1.0.0",
 ]
+
+[project.optional-dependencies]
+test = [
+    "pytest>=8.0.0",
+    "pytest-cov>=4.1.0",
+]
+
+[tool.pytest.ini_options]
+testpaths = ["../tests"]
+pythonpath = ["."]
+python_files = ["test_*.py"]
+python_classes = ["Test*"]
+python_functions = ["test_*"]
+addopts = "-v --tb=short"
+filterwarnings = [
+    "ignore::DeprecationWarning",
+]