MAHOUT-679 : Fix draw_circuit() to return visualization string instead of None

[shiavm006]

Purpose of PR

Fix draw_circuit() method to return visualization string instead of None. The method was printing the circuit visualization but returning None, which contradicted the docstring that claimed it returns a visualization object. Now all backends (Qiskit, Cirq, Amazon Braket) return the visualization string as documented, allowing users to capture and use the return value.

Related Issues or PRs

• Closes [Bug] draw() API returns None instead of circuit visualization object #679

Changes Made

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

Breaking Changes

• Yes
• No

Checklist

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

[guan404ming]

LGTM