feat(agent): config struct avec valeurs par défaut MQTT + tests

TDD : 3 tests d'intégration écrits en premier (config_test.rs), puis
implémentation MqttConfig avec default_mqtt_host/port/topic_base pour
garantir des valeurs par défaut même si la section [protocols.mqtt] est
minimale dans le TOML.

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

agent/src/config.rs

@@ -1,4 +1,3 @@
-// stub — implémenté dans la Task 2
 use serde::Deserialize;
 use std::path::Path;
 
@@ -30,24 +29,51 @@ pub struct UdpConfig {
     pub enabled: bool,
 }
 
-#[derive(Deserialize, Debug, Clone, Default)]
+#[derive(Deserialize, Debug, Clone)]
 pub struct MqttConfig {
     #[serde(default)]
     pub enabled: bool,
-    #[serde(default)]
+    #[serde(default = "default_mqtt_host")]
     pub host: String,
-    #[serde(default)]
+    #[serde(default = "default_mqtt_port")]
     pub port: u16,
-    #[serde(default)]
+    #[serde(default = "default_topic_base")]
     pub topic_base: String,
-    #[serde(default)]
+    #[serde(default = "default_true")]
     pub auto_discovery: bool,
-    #[serde(default)]
+    #[serde(default = "default_true")]
     pub birth_message: bool,
-    #[serde(default)]
+    #[serde(default = "default_true")]
     pub last_will: bool,
 }
 
+impl Default for MqttConfig {
+    fn default() -> Self {
+        MqttConfig {
+            enabled: false,
+            host: default_mqtt_host(),
+            port: default_mqtt_port(),
+            topic_base: default_topic_base(),
+            auto_discovery: true,
+            birth_message: true,
+            last_will: true,
+        }
+    }
+}
+
+fn default_mqtt_host() -> String {
+    "10.0.0.3".to_string()
+}
+fn default_mqtt_port() -> u16 {
+    1883
+}
+fn default_topic_base() -> String {
+    "nanometrics/agents".to_string()
+}
+fn default_true() -> bool {
+    true
+}
+
 #[derive(Deserialize, Debug, Clone, Default)]
 pub struct MetricsConfig {
     #[serde(default)]

agent/tests/config_test.rs

@@ -0,0 +1,78 @@
+use std::io::Write;
+use tempfile::NamedTempFile;
+
+#[test]
+fn test_config_parse_complet() {
+    let mut f = NamedTempFile::new().unwrap();
+    write!(f, r#"
+[server]
+ip = "10.0.0.50"
+port = 9999
+
+[protocols.udp]
+enabled = true
+
+[protocols.mqtt]
+enabled = true
+host = "10.0.0.3"
+port = 1883
+topic_base = "nanometrics/agents"
+auto_discovery = true
+birth_message = true
+last_will = true
+
+[metrics.cpu]
+udp = true
+mqtt = false
+"#).unwrap();
+    let cfg = nanometrics_agent::config::load(f.path()).unwrap();
+    assert_eq!(cfg.server.ip, "10.0.0.50");
+    assert_eq!(cfg.server.port, 9999);
+    assert!(cfg.protocols.udp.enabled);
+    assert!(cfg.protocols.mqtt.enabled);
+    assert_eq!(cfg.protocols.mqtt.host, "10.0.0.3");
+    assert!(cfg.metrics.cpu.udp);
+    assert!(!cfg.metrics.cpu.mqtt);
+}
+
+#[test]
+fn test_config_mqtt_absent() {
+    let mut f = NamedTempFile::new().unwrap();
+    write!(f, r#"
+[server]
+ip = "10.0.0.50"
+port = 9999
+
+[protocols.udp]
+enabled = true
+
+[protocols.mqtt]
+enabled = false
+"#).unwrap();
+    let cfg = nanometrics_agent::config::load(f.path()).unwrap();
+    assert!(!cfg.protocols.mqtt.enabled);
+    // Les valeurs par défaut MQTT doivent être définies
+    assert_eq!(cfg.protocols.mqtt.host, "10.0.0.3");
+    assert_eq!(cfg.protocols.mqtt.port, 1883);
+    assert_eq!(cfg.protocols.mqtt.topic_base, "nanometrics/agents");
+}
+
+#[test]
+fn test_config_metrics_par_defaut() {
+    let mut f = NamedTempFile::new().unwrap();
+    write!(f, r#"
+[server]
+ip = "10.0.0.1"
+port = 9999
+
+[protocols.udp]
+enabled = true
+
+[protocols.mqtt]
+enabled = false
+"#).unwrap();
+    let cfg = nanometrics_agent::config::load(f.path()).unwrap();
+    // Metrics absentes => false par défaut
+    assert!(!cfg.metrics.cpu.udp);
+    assert!(!cfg.metrics.disk.mqtt);
+}