Improve test coverage for qiskit_backend.py to 99% (#1112)

* test: add fallback test for qiskit custom simulator

* test: add coverage for draw_circuit method across all backends

* test: add coverage for apply_swap_gate function across all backends

Author: Howardisme

testing/qumat/test_create_circuit.py

@@ -194,3 +194,33 @@ def test_missing_backend_options_raises_keyerror(self):
 
         with pytest.raises(KeyError, match="missing required key.*backend_options"):
             QuMat(config)
+
+    def test_qiskit_custom_simulator_fallback(self):
+        """Test that Qiskit backend accepts simulator types outside the legacy map."""
+        config = {
+            "backend_name": "qiskit",
+            "backend_options": {"simulator_type": "matrix_product_state"},
+        }
+
+        qumat = QuMat(config)
+
+        assert qumat.backend is not None
+        assert qumat.backend.options.method == "matrix_product_state"
+
+
+@pytest.mark.parametrize("backend_name", TESTING_BACKENDS)
+class TestCircuitDrawing:
+    """Test class for circuit drawing functionality."""
+
+    def test_draw_circuit_returns_output(self, backend_name):
+        """Test that draw_circuit runs successfully and returns an output."""
+        backend_config = get_backend_config(backend_name)
+        qumat = QuMat(backend_config)
+
+        qumat.create_empty_circuit(num_qubits=2)
+        qumat.apply_hadamard_gate(0)
+        qumat.apply_cnot_gate(0, 1)
+
+        drawing = qumat.draw_circuit()
+
+        assert drawing is not None

testing/qumat/test_multi_qubit_gates.py

@@ -490,3 +490,19 @@ def test_cnot_cross_backend_consistency(self, backend_name):
                 assert abs(probabilities[i] - probabilities[j]) < 0.05, (
                     f"Backends have inconsistent CNOT results: {results_dict}"
                 )
+
+
+@pytest.mark.parametrize("backend_name", TESTING_BACKENDS)
+def test_apply_swap_gate(backend_name):
+    """Test that the SWAP gate applies correctly across backends."""
+    backend_config = get_backend_config(backend_name)
+    qumat = QuMat(backend_config)
+
+    qumat.create_empty_circuit(num_qubits=2)
+
+    qumat.apply_pauli_x_gate(0)
+
+    qumat.apply_swap_gate(0, 1)
+
+    results = qumat.execute_circuit()
+    assert results is not None