Implement tests for string transformation functions

Add unit tests for min cost string conversion functions.

Author: ayushrusiya9

test_min_cost_string_conversion.py

@@ -0,0 +1,17 @@
+import pytest
+from strings.min_cost_string_conversion import compute_transform_tables, assemble_transformation
+
+def test_empty_strings():
+    costs, ops = compute_transform_tables("", "", 1,1,1,1)
+    assert costs == [[0]]
+    assert assemble_transformation(ops, 0, 0) == []
+
+def test_copy_only():
+    costs, ops = compute_transform_tables("abc", "abc", 1,2,3,3)
+    assert costs[-1][-1] == 3  # cost = sum of copy costs
+    assert assemble_transformation(ops, len(ops)-1, len(ops[0])-1) == ["Ca", "Cb", "Cc"]
+
+def test_insert_only():
+    costs, ops = compute_transform_tables("", "xyz", 1,1,1,1)
+    seq = assemble_transformation(ops, 0, len(ops[0])-1)
+    assert seq == ["Ix", "Iy", "Iz"]