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/config_server_api.rs

@@ -0,0 +1,140 @@
+use std::ptr;
+
+use easytier_ffi::{
+    in_config_server_callback, is_config_server_client_connected, start_config_server_client,
+    stop_config_server_client,
+};
+use jni::JNIEnv;
+use jni::objects::{JClass, JObject, JString};
+use jni::sys::{JNI_FALSE, JNI_TRUE, jboolean, jint};
+
+use crate::{
+    callback, error,
+    strings::{jstring_to_cstring, optional_jstring_to_cstring},
+};
+
+pub(crate) fn start_config_server_client_jni(
+    env: &mut JNIEnv,
+    config_server_url: JString,
+    hostname: JString,
+    machine_id: JString,
+    secure_mode: jboolean,
+    callback_obj: JObject,
+) -> jint {
+    if in_config_server_callback() {
+        error::throw_exception(
+            env,
+            "Cannot start config server client from config server callback",
+        );
+        return -1;
+    }
+
+    let config_server_url = match jstring_to_cstring(env, &config_server_url) {
+        Ok(cstr) => cstr,
+        Err(e) => {
+            error::throw_exception(env, &format!("Invalid config server URL: {}", e));
+            return -1;
+        }
+    };
+    let hostname = match optional_jstring_to_cstring(env, &hostname) {
+        Ok(cstr) => cstr,
+        Err(e) => {
+            error::throw_exception(env, &format!("Invalid hostname: {}", e));
+            return -1;
+        }
+    };
+    let machine_id = match jstring_to_cstring(env, &machine_id) {
+        Ok(cstr) => cstr,
+        Err(e) => {
+            error::throw_exception(env, &format!("Invalid machine ID: {}", e));
+            return -1;
+        }
+    };
+
+    let callback_ref = if callback_obj.is_null() {
+        None
+    } else {
+        match callback::new_callback(env, &callback_obj) {
+            Ok(state) => Some(state),
+            Err(e) => {
+                error::throw_exception(env, &e);
+                return -1;
+            }
+        }
+    };
+
+    let mut callback_guard = match callback::lock_callback_storage() {
+        Ok(guard) => guard,
+        Err(e) => {
+            error::throw_exception(env, &e);
+            return -1;
+        }
+    };
+    if callback_guard.is_none() {
+        error::clear_callback_error();
+    }
+
+    let callback_fn = callback::callback_fn(&callback_ref);
+    let user_data = callback::user_data(&callback_ref);
+    let result = unsafe {
+        start_config_server_client(
+            config_server_url.as_ptr(),
+            hostname
+                .as_ref()
+                .map(|value| value.as_ptr())
+                .unwrap_or(ptr::null()),
+            machine_id.as_ptr(),
+            secure_mode == JNI_TRUE,
+            callback_fn,
+            user_data,
+        )
+    };
+    if result != 0 {
+        if let Some(error_msg) = error::get_last_error() {
+            error::throw_exception(env, &error_msg);
+        }
+        return result;
+    }
+
+    *callback_guard = callback_ref;
+    result
+}
+
+pub(crate) fn stop_config_server_client_jni(mut env: JNIEnv, _class: JClass) -> jint {
+    if in_config_server_callback() {
+        let result = stop_config_server_client();
+        if result != 0
+            && let Some(error_msg) = error::get_last_error()
+        {
+            error::throw_exception(&mut env, &error_msg);
+        }
+        return result;
+    }
+
+    let mut callback_guard = match callback::lock_callback_storage() {
+        Ok(guard) => guard,
+        Err(e) => {
+            error::throw_exception(&mut env, &e);
+            return -1;
+        }
+    };
+
+    let result = stop_config_server_client();
+    if result != 0 {
+        if let Some(error_msg) = error::get_last_error() {
+            error::throw_exception(&mut env, &error_msg);
+        }
+        return result;
+    }
+
+    *callback_guard = None;
+    result
+}
+
+pub(crate) fn is_config_server_client_connected_jni(_env: JNIEnv, _class: JClass) -> jboolean {
+    if is_config_server_client_connected() != 0 {
+        JNI_TRUE
+    } else {
+        JNI_FALSE
+    }
+}