feat: add POST /format endpoint

test_main.py

@@ -0,0 +1,18 @@
+from fastapi.testclient import TestClient
+
+from main import app
+
+client = TestClient(app)
+
+
+def test_format_endpoint_formats_python_code():
+    response = client.post("/format", json={"code": "x=1\n", "language": "python"})
+    assert response.status_code == 200
+    assert response.json() == {"code": "x = 1\n"}
+
+
+def test_format_endpoint_returns_400_on_syntax_error():
+    response = client.post(
+        "/format", json={"code": "def foo(:\n", "language": "python"}
+    )
+    assert response.status_code == 400