refactor: Add comprehensive type hints to backend modules

[kartikeyg0104]

Overview

This PR adds comprehensive type annotations to all three backend modules, improving IDE support and enabling static type checking. closes #643

What's Changed

• Added type annotations to all functions in qumat/qiskit_backend.py
• Added type annotations to all functions in qumat/cirq_backend.py
• Added type annotations to all functions in qumat/amazon_braket_backend.py
• Added necessary imports (typing.Any, numpy)
• Used modern union syntax for flexible types (e.g., LocalSimulator | AwsDevice)

Why This Change

The backend modules lacked type hints, making it harder for developers to:

• Get proper IDE autocomplete and IntelliSense
• Catch type-related bugs during development
• Understand function signatures at a glance

This change improves developer experience without altering any functionality.

Benefits

• ✅ Better IDE autocomplete and IntelliSense support
• ✅ Enable static type checking with mypy
• ✅ Improved code documentation through types
• ✅ Easier to catch type-related bugs early

Testing

• ✅ All existing tests pass (no functionality changes)
• ✅ Type annotations are consistent across all backends
• ✅ No runtime behavior changes