Fix: Resolved issue with broken IME composing rect in Windows desktop…

… app (#2239)

- Fixed issue where the composing rect would break during IME conversion.
- When the composing region is unknown, the offset is now calculated based on the current selection position.
- Refactored the function to calculate the ComposingRange into a separate function.

lib/src/editor/raw_editor/raw_editor_state_text_input_client_mixin.dart

@@ -97,9 +97,19 @@ mixin RawEditorStateTextInputClientMixin on EditorState
     _textInputConnection!.show();
   }
 
+  TextRange _getComposingRange() {
+    if (_lastKnownRemoteTextEditingValue != null &&
+        _lastKnownRemoteTextEditingValue?.composing.isValid == true) {
+      return _lastKnownRemoteTextEditingValue!.composing;
+    } else if (textEditingValue.composing.isValid == true) {
+      return textEditingValue.composing;
+    } else {
+      return widget.controller.selection;
+    }
+  }
+
   void _updateComposingRectIfNeeded() {
-    final composingRange = _lastKnownRemoteTextEditingValue?.composing ??
-        textEditingValue.composing;
+    final composingRange = _getComposingRange();
     if (hasConnection) {
       assert(mounted);
       final offset = composingRange.isValid ? composingRange.start : 0;