fix: resolve CI issues - push complete implementation with tests

tests/conftest.py

@@ -1,5 +1,3 @@
-"""Pytest configuration and fixtures for gdiffer tests."""
-
 import sys
 from pathlib import Path
 
@@ -26,6 +24,21 @@ index 1234567..89abcde 100644
  """
 
 
+@pytest.fixture
+def python_diff():
+    return """diff --git a/utils.py b/utils.py
+index abc123..def456 100644
+--- a/utils.py
++++ b/utils.py
+@@ -5,8 +5,10 @@ def calculate(a, b):
+     result = a + b
+     return result
+
++def multiply(a, b):
++    return a * b
+ """
+
+
 @pytest.fixture
 def multi_file_diff():
     return """diff --git a/app.py b/app.py