ut for response_handlers.py

[tukwila]

Summary

1. prompt response_handlers.py ut coverage from 30% to 85%
2. enhance dict value is None

Details

• [ ]

Test Plan

python -m pytest tests/unit/backends/test_response_handlers.py

Related Issues

• Resolves #

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• "I certify that all code in this PR is my own, except as noted below."

Use of AI

• Includes AI-assisted code completion
• Includes code generated by an AI application
• Includes AI-generated tests (NOTE: AI written tests should have a docstring that includes ## WRITTEN BY AI ##)