fix: update codecov action to v4 to resolve GPG verification error

Update codecov/codecov-action from v5 to v4 to fix GPG signature verification
failures in coverage upload step. v4 resolves the GPG key verification issue
that was causing CI failures.

Fixes: 'gpg: Can't check signature: No public key' error in PR tests coverage upload

Author: Nac77

.github/workflows/_test.yml

@@ -104,7 +104,7 @@ jobs:
 
       - name: Upload coverage to Codecov
         if: inputs.with-coverage
-        uses: codecov/codecov-action@v5
+        uses: codecov/codecov-action@v4
         with:
           directory: ./coverage/reports/
           env_vars: PYTHON