Add comprehensive tests for OAuth callback flows and update pyproject.toml

- Extended OAuth callback tests to cover various scenarios (e.g., account linking, user creation, inactive users, and token/user info failures).
- Added `app/init_db.py` to the excluded files in `pyproject.toml`.

backend/pyproject.toml

@@ -326,6 +326,7 @@ omit = [
     "*/__pycache__/*",
     "*/alembic/versions/*",
     "*/.venv/*",
+    "app/init_db.py",  # CLI script for database initialization
 ]
 branch = true