Fix JSON results streaming without readlines

[J0JIN]

Summary

• Iterate over JSON result streams directly instead of calling readlines()
• Add a unit test to ensure JSONResultsReader does not require readlines()

Testing

• uv --no-config run python -m pytest --no-header --ff -ra -vv ./tests/unit/test_results.py
• uv --no-config run basedpyright tests/unit/test_results.py splunklib/results.py

[Ickerday]

Hi, this is obviously vibe-coded, so what's the motivation behind this? Have you considered what impact this would have on existing SDK users? Additionally - what value do you see in this unit test?