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-contrib/easytier-android-jni/src/callback.rs

@@ -0,0 +1,124 @@
+use std::{
+    ffi::{CStr, c_char, c_void},
+    sync::{Arc, Mutex, MutexGuard},
+};
+
+use easytier_ffi::ConfigServerEventCallback;
+use jni::JNIEnv;
+use jni::objects::{GlobalRef, JObject, JValue};
+use once_cell::sync::Lazy;
+
+use crate::error;
+
+pub(crate) struct JniConfigServerCallback {
+    java_vm: jni::JavaVM,
+    callback: GlobalRef,
+}
+
+static CONFIG_SERVER_CALLBACK: Lazy<Mutex<Option<Arc<JniConfigServerCallback>>>> =
+    Lazy::new(|| Mutex::new(None));
+
+pub(crate) fn lock_callback_storage()
+-> Result<MutexGuard<'static, Option<Arc<JniConfigServerCallback>>>, String> {
+    CONFIG_SERVER_CALLBACK
+        .lock()
+        .map_err(|e| format!("Failed to lock config server callback: {}", e))
+}
+
+pub(crate) fn new_callback(
+    env: &mut JNIEnv,
+    callback: &JObject,
+) -> Result<Arc<JniConfigServerCallback>, String> {
+    let java_vm = env
+        .get_java_vm()
+        .map_err(|e| format!("Failed to get JavaVM: {:?}", e))?;
+    let callback = env
+        .new_global_ref(callback)
+        .map_err(|e| format!("Failed to create callback global ref: {:?}", e))?;
+    Ok(Arc::new(JniConfigServerCallback { java_vm, callback }))
+}
+
+pub(crate) fn callback_fn(
+    callback: &Option<Arc<JniConfigServerCallback>>,
+) -> ConfigServerEventCallback {
+    callback
+        .as_ref()
+        .map(|_| config_server_event_callback as unsafe extern "C" fn(*const c_char, *mut c_void))
+}
+
+pub(crate) fn user_data(callback: &Option<Arc<JniConfigServerCallback>>) -> *mut c_void {
+    callback
+        .as_ref()
+        .map(|callback| Arc::as_ptr(callback) as *mut c_void)
+        .unwrap_or(std::ptr::null_mut())
+}
+
+impl JniConfigServerCallback {
+    fn clear_pending_exception(
+        env: &mut JNIEnv,
+        context: &str,
+        error: &dyn std::fmt::Debug,
+    ) -> String {
+        match env.exception_check() {
+            Ok(true) => {
+                if let Err(clear_err) = env.exception_clear() {
+                    return format!(
+                        "{}: {:?}; failed to clear pending Java exception: {:?}",
+                        context, error, clear_err
+                    );
+                }
+            }
+            Ok(false) => {}
+            Err(check_err) => {
+                return format!(
+                    "{}: {:?}; failed to check pending Java exception: {:?}",
+                    context, error, check_err
+                );
+            }
+        }
+
+        format!("{}: {:?}", context, error)
+    }
+
+    fn on_event(&self, event_json: *const c_char) -> Result<(), String> {
+        let event_json = unsafe { CStr::from_ptr(event_json) }
+            .to_str()
+            .map_err(|e| format!("Invalid config server event JSON: {:?}", e))?;
+        let mut env = self
+            .java_vm
+            .attach_current_thread()
+            .map_err(|e| format!("Failed to attach callback thread: {:?}", e))?;
+        let event_json = env.new_string(event_json).map_err(|e| {
+            Self::clear_pending_exception(&mut env, "Failed to create event string", &e)
+        })?;
+
+        if let Err(e) = env.call_method(
+            self.callback.as_obj(),
+            "onEvent",
+            "(Ljava/lang/String;)V",
+            &[JValue::from(&event_json)],
+        ) {
+            return Err(Self::clear_pending_exception(
+                &mut env,
+                "Failed to call config server callback",
+                &e,
+            ));
+        }
+        Ok(())
+    }
+}
+
+unsafe extern "C" fn config_server_event_callback(
+    event_json: *const c_char,
+    user_data: *mut c_void,
+) {
+    if event_json.is_null() || user_data.is_null() {
+        return;
+    }
+
+    let callback = unsafe { &*(user_data as *const JniConfigServerCallback) };
+
+    if let Err(error) = callback.on_event(event_json) {
+        error::set_callback_error(error);
+    }
+}