ci: restore pytest-cov dev dependency

Accidentally dropped along with pytest-bdd in the previous refactor. The
"Run tests with coverage" CI step depends on it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mcdax

poetry.lock

@@ -31,6 +31,7 @@ httpx = "^0.27.0"
 [tool.poetry.group.dev.dependencies]
 pytest = "^8.0.0"
 pytest-asyncio = "^0.23.0"
+pytest-cov = "^4.0.0"
 pytest-mock = "^3.12.0"
 black = "^24.0.0"
 mypy = "^1.8.0"