feat: TYPE_LABEL = "Storage" replaces storage_device = True flag

The TYPE_LABEL attribute on the base class hierarchy is now the
canonical way for drivers to declare their device category. The
storage_device sentinel flag is removed — classification is determined
by reading TYPE_LABEL via getattr() in the backend.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_omv_type_label.py

@@ -0,0 +1,18 @@
+"""Tests for TYPE_LABEL on OpenMediaVaultDriver."""
+
+from napalm_openmediavault.openmediavault import OpenMediaVaultDriver
+
+
+def test_type_label_is_storage():
+    assert OpenMediaVaultDriver.TYPE_LABEL == "Storage"
+
+
+def test_type_label_overrides_linux():
+    from napalm_linux.linux import LinuxDriver
+    assert LinuxDriver.TYPE_LABEL == "Linux"
+    assert OpenMediaVaultDriver.TYPE_LABEL != LinuxDriver.TYPE_LABEL
+
+
+def test_storage_device_flag_removed():
+    assert not hasattr(OpenMediaVaultDriver, "storage_device"), \
+        "storage_device is superseded by TYPE_LABEL = 'Storage'"