ci: install integration requirements in the HA test job

Every tests_ha test failed at setup with reason='Import error'. The
integration itself imported fine - config_flow.py did not, because it
imports `serial` and `serial.tools.list_ports` at module level.

pytest-homeassistant-custom-component does not install a custom
integration's manifest.json requirements; Home Assistant does that
itself at runtime. So the harness had no pymodbus or pyserial, and
HA's platform loader reported the failure only as the opaque
"Platform growatt_modbus.config_flow not found".

The unit job already installed both explicitly. This matches it.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: 0xAHA

.github/workflows/tests.yaml

@@ -44,8 +44,12 @@ jobs:
         with:
           python-version: "3.13"
 
+      # pymodbus and pyserial are the integration's own manifest.json requirements.
+      # Home Assistant installs those itself at runtime, but the test harness does not —
+      # so without them config_flow.py fails at `import serial` and every test dies with
+      # a bare "Import error" from HA's platform loader.
       - name: Install Home Assistant test harness
-        run: pip install pytest-homeassistant-custom-component==0.13.316
+        run: pip install pytest-homeassistant-custom-component==0.13.316 pymodbus pyserial
 
       - name: Run tests
         run: pytest tests_ha/ -v