Improve test coverage for qiskit_backend.py to 99%

[Howardisme]

Related Issues

Related to #1058

Changes

• Bug fix
• New feature
• Refactoring
• Documentation
• Test
• CI/CD pipeline
• Other

Why

This PR improves the test coverage for the qiskit_backend.py module by addressing few previously untested code paths.
and also improves some test coverage for all backends(cirq_backend.py, amazon_braket_backend.py).

How

Add unit tests covering:

• Backend Initialization (add test_qiskit_custom_simulator_fallback):

  • Triggered the else block of initialize_backend using a mock setup ("simulator_type": "matrix_product_state"). Evaluates the correct propagation of the configuration parameter through to the underlying AerSimulator.

• Circuit Visualization (add test_draw_circuit_returns_output):

  • Validated the rendering function (draw_circuit) in multiple backend configurations by creating class TestCircuitDrawing. Testing all backends as well.

• Swap Operation (add test_apply_swap_gate):

  • Tested the swap gate function (apply_swap_gate) by initiating state (|10⟩ by a Pauli-X), executing apply_swap_gate, and then checked the operation returned successfully in qumat execution contexts. Testing all backends as well.

Coverage Improve:

• qiskit_backend.py: 95% → 99%
• amazon_braket_backend.py: 99% → 100%
• cirq_backend.py: 93% → 97%
• qumat.py: 92% → 97%

Checklist

• Added or updated unit tests for all changes
• Added or updated documentation for all changes