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

src/caesar_cipher/cipher.py

@@ -1,14 +1,21 @@
 ## @file cipher.py
-#  @brief Caesar cipher encryption and decryption.
+#  @brief Caesar cipher: one shared function for encoding and decoding.
 
-from caesar_cipher.constants import ALPHABET
+from caesar_cipher.constants import ALPHABET, DIRECTION_DECODE, DIRECTION_ENCODE
 
 
-def encrypt(text: str, shift: int) -> str:
-    ## @brief Encrypt a text with the Caesar cipher.
-    #  @param text  Original text.
-    #  @param shift Positions to move each letter (may be negative or > 26).
-    #  @return The encrypted text; non-letters are kept, case is preserved.
+def caesar(text: str, shift: int, direction: str) -> str:
+    ## @brief Encode or decode a text with the Caesar cipher.
+    #  @param text      Text to transform.
+    #  @param shift     Positions to move each letter (may be negative or > 26).
+    #  @param direction DIRECTION_ENCODE or DIRECTION_DECODE.
+    #  @return The transformed text; non-letters are kept, case is preserved.
+    #  @throws ValueError If direction is not a known direction.
+    if direction == DIRECTION_DECODE:
+        shift = -shift
+    elif direction != DIRECTION_ENCODE:
+        raise ValueError(f"Unknown direction: {direction!r}")
+
     result = []
     for char in text:
         lower = char.lower()
@@ -21,9 +28,17 @@ def encrypt(text: str, shift: int) -> str:
     return "".join(result)
 
 
+def encrypt(text: str, shift: int) -> str:
+    ## @brief Encrypt a text with the Caesar cipher.
+    #  @param text  Original text.
+    #  @param shift Number of positions to shift forward.
+    #  @return The encrypted text.
+    return caesar(text, shift, DIRECTION_ENCODE)
+
+
 def decrypt(text: str, shift: int) -> str:
     ## @brief Decrypt a Caesar-cipher text.
     #  @param text  Encrypted text.
     #  @param shift Shift that was used to encrypt.
     #  @return The original text.
-    return encrypt(text, -shift)
+    return caesar(text, shift, DIRECTION_DECODE)