add basic tests for bad module creation

internal/module/test/mqtt-client_test.go

@@ -35,3 +35,96 @@ func TestMQTTClientFromRegistry(t *testing.T) {
 		t.Fatalf("mqtt.client module has wrong type: %s", moduleInstance.Type())
 	}
 }
+
+func TestBadMQTTClient(t *testing.T) {
+	tests := []struct {
+		name        string
+		params      map[string]any
+		errorString string
+	}{
+		{
+			name: "no broker param",
+			params: map[string]any{
+				"topic":    "test/topic",
+				"clientId": "test",
+			},
+			errorString: "mqtt.client requires a broker parameter",
+		},
+		{
+			name: "non-string broker",
+			params: map[string]any{
+				"broker":   123,
+				"topic":    "test/topic",
+				"clientId": "test",
+			},
+			errorString: "mqtt.client broker must be a string",
+		},
+		{
+			name: "no topic param",
+			params: map[string]any{
+				"broker":   "mqtt://localhost:1883",
+				"clientId": "test",
+			},
+			errorString: "mqtt.client requires a topic parameter",
+		},
+		{
+			name: "non-string topic",
+			params: map[string]any{
+				"broker":   "mqtt://localhost:1883",
+				"topic":    123,
+				"clientId": "test",
+			},
+			errorString: "mqtt.client topic must be a string",
+		},
+		{
+			name: "no clientId param",
+			params: map[string]any{
+				"broker": "mqtt://localhost:1883",
+				"topic":  "test/topic",
+			},
+			errorString: "mqtt.client requires a clientId parameter",
+		},
+		{
+			name: "non-string clientId",
+			params: map[string]any{
+				"broker":   "mqtt://localhost:1883",
+				"topic":    "test/topic",
+				"clientId": 123,
+			},
+			errorString: "mqtt.client clientId must be a string",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+
+			registration, ok := module.ModuleRegistry["mqtt.client"]
+			if !ok {
+				t.Fatalf("mqtt.client module not registered")
+			}
+
+			moduleInstance, err := registration.New(config.ModuleConfig{
+				Id:     "test",
+				Type:   "mqtt.client",
+				Params: test.params,
+			})
+
+			if err != nil {
+				if test.errorString != err.Error() {
+					t.Fatalf("mqtt.client got error '%s', expected '%s'", err.Error(), test.errorString)
+				}
+				return
+			}
+
+			err = moduleInstance.Start(t.Context())
+
+			if err == nil {
+				t.Fatalf("mqtt.client expected to fail")
+			}
+
+			if err.Error() != test.errorString {
+				t.Fatalf("mqtt.client got error '%s', expected '%s'", err.Error(), test.errorString)
+			}
+		})
+	}
+}