MAHOUT-744 Improve error messages for missing backend_config keys

[viiccwen]

Purpose of PR

Improve error handling in QuMat.__init__() to provide clear and helpful error messages when backend_config is missing required keys (backend_name or backend_options).

Previously, the code would raise a generic KeyError without clear indication of what was missing or how to fix it, making debugging difficult for users.

Related Issues or PRs

resolve #800

Changes Made

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

Breaking Changes

• Yes
• No

Changes Summary

Code Changes:

• Added validation in QuMat.__init__() to check if backend_config is a dictionary
• Added validation to check for required keys (backend_name and backend_options) before accessing them

Test Changes:

• Added TestBackendConfigValidation class in test_create_circuit.py
• Added tests for invalid type (non-dictionary config)
• Added tests for missing backend_name key
• Added tests for missing backend_options key

Before:

# Would raise: KeyError: 'backend_name' (not helpful)
qumat = QuMat({"backend_options": {...}})

After:

# Raises: KeyError: "backend_config is missing required key 'backend_name'. 
#          Please provide a backend name (e.g., 'qiskit', 'cirq', 'amazon_braket')"
qumat = QuMat({"backend_options": {...}})

Checklist

• Added or updated unit tests for all changes
• Added or updated documentation for all changes (updated docstring)
• Successfully built and ran all unit tests or manual tests locally
• PR title follows "MAHOUT-XXX: Brief Description" format (if related to an issue)
• Code follows ASF guidelines

[viiccwen]

The description of error messages could adjust if there is anything unclear.