add test case for dd_llmobs_disabled in test_llmobs_service.py

[Gekko0114]

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As I mentioned in this issue #10859, LLM observability data is being sent even though DD_LLMOBS_ENABLED is set to "0".
To reproduce this issue, I created a test case.
Since I am not familiar with dd-trace-py codebase, this code might not be correct.
Please let me know if I have misunderstood anything.