add test case for dd_llmobs_disabled in test_llmobs_service.py

[Gekko0114]

Checklist

• [ ] PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• [ ] Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

As I mentioned in this issue https://github.com/DataDog/dd-trace-py/issues/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.