add openai/resolve() ut to cover streaming

[tukwila]

Summary

Add new ut about resolve() streaming case in src/guidellm/backends/openai.py to prompt ut coverage from 73% to 89%:

before:

---------- coverage: platform darwin, python 3.11.5-final-0 ----------
Name                                                                                       Stmts   Miss  Cover   Missing
------------------------------------------------------------------------------------------------------------------------
.tox/test/lib/python3.11/site-packages/guidellm/backends/openai.py                           122     33    73%   164, 167, 187, 228, 237, 267-309, 330, 336, 341-355

After:

.tox/test/lib/python3.11/site-packages/guidellm/backends/openai.py                           122     13    89%   164, 167, 187, 228, 237, 330, 336, 341-355

ut cmd:
python -m pytest tests/unit/backends/test_openai_backend.py

Details

• [ ]

Test Plan

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 ##)

[sjmonson]

Needs to be rebased due to conflict