SK-2872: restore coverage to requirements.txt for CI test runner

coverage was correctly removed from setup.py install_requires (not a
production dep) but also incorrectly removed from requirements.txt.
CI runs 'python -m coverage run' so it must be present as a dev/CI dep.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: saileshwar-skyflow

requirements.txt

@@ -5,3 +5,4 @@ requests ~= 2.32.4
 cryptography >= 44.0.2
 httpx >= 0.28.1
 python-dotenv >= 1.1.0, < 2
+coverage >= 7.8.0