Fix(reasoning_gym/games/countdown): Resolve SymPy parsing conflict for 10+ input numbers (#514)

* Refactor expression generation and substitution logic

Updated symbol naming and added safe replacement for expressions.

* Add expr_str to return values in countdown.py

Modified return statement to include the modified expression string.

* Implement test for min_numbers exceeding 10

Add test for CountdownDataset with more than 10 numbers

* Remove trailing-whitespace

* Improve readability of CountdownDataset initialization

Refactor CountdownDataset initialization for readability.

tests/test_countdown.py

@@ -114,6 +114,16 @@ def test_edge_cases_2():
     assert dataset.score_answer(answer=answer, entry=item) != 1.0
 
 
+def test_countdown_more_numbers():
+    """Test when min_numbers exceed 10"""
+    dataset = CountdownDataset(
+        CountdownConfig(min_numbers=11, max_numbers=11, shuffle=False, size=5, seed=42)
+    )  # Set 11 engaged numbers for testing
+
+    for item in dataset:
+        assert item["metadata"]["target"] == int(eval(item["metadata"]["expression"]))
+
+
 def test_countdown_game_randomization():
     """Test number randomization configuration"""
     config = CountdownConfig(min_numbers=4, max_numbers=4, shuffle=False, size=10, seed=42)  # Fixed size for testing