test: migrate to pytest

Replace unittest-based coverage with pytest and update CI/docs to run the new suite.

pyproject.toml

@@ -27,9 +27,14 @@ ctfd-mcp = "ctfd_mcp.main:main"
 [dependency-groups]
 dev = [
     "pre-commit>=3.6.0",
+    "pytest>=8.0.0",
     "ruff>=0.5.0",
 ]
 
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["src", "."]
+
 [tool.ruff]
 line-length = 88
 target-version = "py313"