test: Add test for sending response after sending complete final flag

[kthui]

What does the PR do?

Enhance existing test on response sender when the model would sent additional response(s) after sending the complete final flag. The additional response(s) should cause an exception to be raised.

This test enhancement is due to deleting the response factory after the response sender has sent the complete final flag, to ensure the model cannot accidentally trigger a dereference on the deleted response factory by sending response(s) after sending complete final flag.

Checklist

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• build
• ci
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• feat
• fix
• perf
• refactor
• revert
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• test

Related PRs:

triton-inference-server/python_backend#373
triton-inference-server/vllm_backend#55

Where should the reviewer start?

Start with the related PRs before coming back to this one.

Test plan:

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• CI Pipeline ID: 17156719

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