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_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