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
@@ -56,7 +69,7 @@ index abc123..xyz789 100644
      return True
 +
 +def new_func(): pass
-"""
+ """
 
 
 @pytest.fixture
@@ -66,7 +79,7 @@ def sql_injection_diff():
      query = "SELECT * FROM users WHERE name = '" + username + "'"
      return execute_query(query)
 +    query = "SELECT * FROM users WHERE id = " + user_id
-"""
+ """
 
 
 @pytest.fixture