feat: SNMP support — get_snmp_config(), fix_snmp for Smart switches

NetgearPlus: get_snmp_config() returns None (SNMP not accessible via HTTP API).
NetgearSmart: get_snmp_config() reads current community, fix_snmp configures
'snmp-server community public ro' via CLI.

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

napalm_netgear/netgear_plus.py

@@ -730,6 +730,13 @@ class NetgearPlusDriver(SwitchDriver):
             "set_interface() is not supported for Netgear Plus switches."
         )
 
+    # ── SNMP / Health ──────────────────────────────────────────────────────────
+
+    def get_snmp_config(self):
+        """SNMP is not accessible via the HTTP API used by this driver.
+        Returns None to indicate SNMP health monitoring is not supported."""
+        return None
+
     def get_device_warnings(self) -> list:
         """Check for available firmware updates and return a warning if the
         device is not running the latest known firmware.