Improve coverage for qumat/qdp.py by testing stub fallback behavior

[alisha-1000]

Related Issues

N/A

Changes

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

Why

qumat/qdp.py provides a fallback stub implementation when the native _qdp
extension is not available. This behavior is important for environments
without GPU/native builds, but it was not explicitly covered by tests.

Adding dedicated tests ensures that the fallback mechanism works as expected
and remains stable over time.

How

• Added testing/qumat/test_qdp_stub.py
• Forced a fresh import of qumat.qdp to trigger the fallback code path
• Verified that:
  • An ImportWarning is emitted when _qdp is unavailable
  • Stub classes (QdpEngine, QuantumTensor) raise ImportError on instantiation
  • __all__ exports are correctly defined

All existing tests pass locally:

• 561 passed
• 91 skipped
• Total coverage: 93% (442 statements, 32 missed)

This change improves coverage for qumat/qdp.py without modifying runtime behavior or affecting the native extension logic.

Checklist

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

[alisha-1000]

Superseded by #1084, which contains a more complete and refined coverage update for qumat/qdp.py.

Closing this PR to avoid duplication and keep the history clean.