Refactor to unified caesar() function with encode/decode support

- Replace separate encrypt/decrypt with shared caesar() function
- Add direction constants (DIRECTION_ENCODE, DIRECTION_DECODE) and
  prompts
- Update main.py to support interactive encode/decode loop
- Keep encrypt/decrypt as wrappers for backward compatibility
- Add tests for caesar() with both directions and error handling
- Update tests for main.py with interactive scenario tests

tests/test_cipher.py

@@ -1,4 +1,6 @@
-from caesar_cipher import decrypt, encrypt
+import pytest
+
+from caesar_cipher import caesar, decrypt, encrypt
 from caesar_cipher.env import load_env
 
 
@@ -37,3 +39,13 @@ def test_decrypt_roundtrip():
 def test_decrypt_wraps_around_alphabet():
     assert decrypt("abc", 3) == "xyz"
     assert decrypt("def", 29) == "abc"
+
+
+def test_caesar_encode_and_decode():
+    assert caesar("hello", 5, "encode") == "mjqqt"
+    assert caesar("mjqqt", 5, "decode") == "hello"
+
+
+def test_caesar_unknown_direction():
+    with pytest.raises(ValueError):
+        caesar("hello", 5, "sideways")

tests/test_main.py

@@ -0,0 +1,28 @@
+from caesar_cipher.main import main
+
+
+def run(monkeypatch, capsys, answers):
+    it = iter(answers)
+    monkeypatch.setattr("builtins.input", lambda _="": next(it))
+    main()
+    return capsys.readouterr().out
+
+
+def test_encode_then_stop(monkeypatch, capsys):
+    out = run(monkeypatch, capsys, ["encode", "hello", "5", "no"])
+    assert "mjqqt" in out and "Goodbye" in out
+
+
+def test_decode_then_go_again(monkeypatch, capsys):
+    out = run(monkeypatch, capsys, ["decode", "mjqqt", "5", "yes", "encode", "a", "1", "no"])
+    assert "hello" in out and "b" in out
+
+
+def test_invalid_input_reprompts(monkeypatch, capsys):
+    out = run(monkeypatch, capsys, ["nope", "decode", "x", "abc", "encode", "a", "1", "no"])
+    assert "encode' or 'decode" in out and "whole number" in out
+
+
+def test_title_printed_plain_when_not_a_tty(monkeypatch, capsys):
+    out = run(monkeypatch, capsys, ["encode", "a", "1", "no"])
+    assert "____" in out and "\033[" not in out