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/error.rs

@@ -0,0 +1,74 @@
+use std::{
+    ffi::{CStr, c_char},
+    ptr,
+    sync::Mutex,
+};
+
+use easytier_ffi::{free_string, get_error_msg};
+use jni::JNIEnv;
+use jni::objects::JClass;
+use jni::sys::jstring;
+use once_cell::sync::Lazy;
+
+static JNI_CALLBACK_ERROR: Lazy<Mutex<Option<String>>> = Lazy::new(|| Mutex::new(None));
+
+pub(crate) fn set_callback_error(error: String) {
+    log::error!("{}", error);
+    if let Ok(mut guard) = JNI_CALLBACK_ERROR.lock() {
+        *guard = Some(error);
+    }
+}
+
+pub(crate) fn clear_callback_error() {
+    if let Ok(mut guard) = JNI_CALLBACK_ERROR.lock() {
+        *guard = None;
+    }
+}
+
+fn take_callback_error() -> Option<String> {
+    JNI_CALLBACK_ERROR
+        .lock()
+        .ok()
+        .and_then(|mut guard| guard.take())
+}
+
+fn get_ffi_last_error() -> Option<String> {
+    unsafe {
+        let mut error_ptr: *const c_char = ptr::null();
+        get_error_msg(&mut error_ptr);
+        if error_ptr.is_null() {
+            None
+        } else {
+            let error_cstr = CStr::from_ptr(error_ptr);
+            let error_str = error_cstr.to_string_lossy().into_owned();
+            free_string(error_ptr);
+            Some(error_str)
+        }
+    }
+}
+
+pub(crate) fn get_last_error() -> Option<String> {
+    match (get_ffi_last_error(), take_callback_error()) {
+        (Some(ffi_error), Some(callback_error)) => Some(format!(
+            "{}; config server callback error: {}",
+            ffi_error, callback_error
+        )),
+        (Some(ffi_error), None) => Some(ffi_error),
+        (None, Some(callback_error)) => Some(callback_error),
+        (None, None) => None,
+    }
+}
+
+pub(crate) fn throw_exception(env: &mut JNIEnv, message: &str) {
+    let _ = env.throw_new("java/lang/RuntimeException", message);
+}
+
+pub(crate) fn get_last_error_jni(env: JNIEnv, _class: JClass) -> jstring {
+    match get_last_error() {
+        Some(error) => match env.new_string(&error) {
+            Ok(jstr) => jstr.into_raw(),
+            Err(_) => ptr::null_mut(),
+        },
+        None => ptr::null_mut(),
+    }
+}