ci: Add --cov-branch flag for Codecov branch coverage tracking

Author: anhmtk

.github/workflows/tests.yml

@@ -51,6 +51,7 @@ jobs:
             --cov=backend.learning \
             --cov=backend.vector_db \
             --cov=backend.validators \
+            --cov-branch \
             --cov-report=xml \
             --cov-report=html \
             --cov-report=term \