pyproject: add pytest-cov to dev deps (CI uses --cov flag)

The tests.yml workflow runs 'pytest --cov=epistemic_forge ...' but
pytest-cov wasn't in [dev] deps. Add it so coverage flags resolve.

Author: faresrafat3

pyproject.toml

@@ -34,7 +34,7 @@ classifiers = [
 dependencies = ["litellm>=1.40.0", "duckduckgo-search>=5.0.0", "chromadb>=0.4.0", "nbformat>=5.0.0", "fastapi>=0.100.0", "uvicorn>=0.20.0", "sse-starlette>=2.0.0", "pydantic>=2.0.0", "loguru>=0.7.0", "tenacity>=8.0.0", "instructor>=1.3.0", "rich>=13.0.0", "streamlit>=1.35.0"]
 
 [project.optional-dependencies]
-dev = ["pytest>=7.0", "pytest-mock", "ruff>=0.1.0", "mypy>=1.0.0", "bandit>=1.7.0"]
+dev = ["pytest>=7.0", "pytest-mock", "pytest-cov>=4.0", "ruff>=0.1.0", "mypy>=1.0.0", "bandit>=1.7.0"]
 
 [project.scripts]
 epistemic-forge = "epistemic_forge.cli:main"