feat(ffi): add config server client bindings (#2320)

Add config server client support for the C FFI and Android JNI bindings.

Reuse the existing easytier::web_client::run_web_client path and 
NetworkInstanceManager; OHOS is unchanged.

Report successful remote config apply/delete operations through a 
callback, with one JSON event per affected instance.

Keep the config server client and FFI data plane mutually exclusive: once 
either side is in use, the other side returns an error instead of sharing
lifecycle state.

easytier/src/instance_manager.rs

@@ -34,6 +34,7 @@ pub struct NetworkInstanceManager {
     instance_error_messages: Arc<DashMap<uuid::Uuid, String>>,
     config_dir: Option<PathBuf>,
     guard_counter: Arc<()>,
+    remote_mutation_lock: Arc<tokio::sync::Mutex<()>>,
 }
 
 impl Default for NetworkInstanceManager {
@@ -51,6 +52,7 @@ impl NetworkInstanceManager {
             instance_error_messages: Arc::new(DashMap::new()),
             config_dir: None,
             guard_counter: Arc::new(()),
+            remote_mutation_lock: Arc::new(tokio::sync::Mutex::new(())),
         }
     }
 
@@ -59,6 +61,10 @@ impl NetworkInstanceManager {
         self
     }
 
+    pub fn remote_mutation_lock(&self) -> Arc<tokio::sync::Mutex<()>> {
+        self.remote_mutation_lock.clone()
+    }
+
     fn start_instance_task(&self, instance_id: uuid::Uuid) -> Result<(), anyhow::Error> {
         if tokio::runtime::Handle::try_current().is_err() {
             return Err(anyhow::anyhow!(
@@ -272,6 +278,12 @@ impl NetworkInstanceManager {
             .map(|instance| instance.value().get_config_file_control().clone())
     }
 
+    pub fn get_instance_config(&self, instance_id: &uuid::Uuid) -> Option<TomlConfigLoader> {
+        self.instance_map
+            .get(instance_id)
+            .map(|instance| instance.value().get_config())
+    }
+
     pub fn get_instance_network_config_source(
         &self,
         instance_id: &uuid::Uuid,