notification.kafka: add SASL authentication and TLS support (#8832)

* notification.kafka: add SASL authentication and TLS support (#8827)

Wire sarama SASL (PLAIN, SCRAM-SHA-256, SCRAM-SHA-512) and TLS
configuration into the Kafka notification producer and consumer,
enabling connections to secured Kafka clusters.

* notification.kafka: validate mTLS config

* kafka notification: validate partial mTLS config, replace panics with errors

- Reject when only one of tls_client_cert/tls_client_key is provided
- Replace three panic() calls in KafkaInput.initialize with returned errors

* kafka notification: enforce minimum TLS 1.2 for Kafka connections

weed/notification/kafka/kafka_sasl_tls_test.go

@@ -0,0 +1,65 @@
+package kafka
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/Shopify/sarama"
+)
+
+func TestConfigureSASLTLSRejectsPartialMTLSConfig(t *testing.T) {
+	tests := []struct {
+		name string
+		cfg  SASLTLSConfig
+	}{
+		{
+			name: "missing key",
+			cfg: SASLTLSConfig{
+				TLSEnabled:    true,
+				TLSClientCert: "/tmp/client.crt",
+			},
+		},
+		{
+			name: "missing cert",
+			cfg: SASLTLSConfig{
+				TLSEnabled:   true,
+				TLSClientKey: "/tmp/client.key",
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ConfigureSASLTLS(sarama.NewConfig(), tt.cfg)
+			if err == nil {
+				t.Fatal("expected error")
+			}
+			if !strings.Contains(err.Error(), "both tls_client_cert and tls_client_key must be provided") {
+				t.Fatalf("unexpected error: %v", err)
+			}
+		})
+	}
+}
+
+func TestConfigureSASLTLSConfiguresSCRAMSHA256(t *testing.T) {
+	config := sarama.NewConfig()
+	err := ConfigureSASLTLS(config, SASLTLSConfig{
+		SASLEnabled:   true,
+		SASLMechanism: "SCRAM-SHA-256",
+		SASLUsername:  "alice",
+		SASLPassword:  "secret",
+	})
+	if err != nil {
+		t.Fatalf("ConfigureSASLTLS returned error: %v", err)
+	}
+
+	if !config.Net.SASL.Enable {
+		t.Fatal("expected SASL to be enabled")
+	}
+	if config.Net.SASL.Mechanism != sarama.SASLTypeSCRAMSHA256 {
+		t.Fatalf("unexpected mechanism: %v", config.Net.SASL.Mechanism)
+	}
+	if config.Net.SASL.SCRAMClientGeneratorFunc == nil {
+		t.Fatal("expected SCRAM client generator")
+	}
+}