fix(transport): filter whitespace-only AI text responses (#759)

Modify `A2uiTransportAdapter` and `A2uiAgentConnector` to trim
incoming text parts from the AI model and discard any that
are empty or consist only of whitespace. This prevents the
UI from rendering empty messages or trailing newlines when the
model responds with whitespace chunks.

Author: gspencergoog

packages/genui/lib/src/transport/a2ui_transport_adapter.dart

@@ -65,7 +65,8 @@ class A2uiTransportAdapter implements Transport {
   Stream<String> get incomingText => _pipeline
       .where((e) => e is TextEvent)
       .cast<TextEvent>()
-      .map((e) => e.text);
+      .map((e) => e.text.trim())
+      .where((text) => text.isNotEmpty);
 
   /// A stream of A2UI messages parsed from the input.
   @override

packages/genui_a2a/lib/src/a2ui_agent_connector.dart

@@ -206,8 +206,9 @@ class A2uiAgentConnector {
               if (part is DataPart) {
                 _processA2uiMessages(part.data);
               } else if (part is TextPart) {
-                if (!_textController.isClosed) {
-                  _textController.add(part.text);
+                final String trimmedText = part.text.trim();
+                if (trimmedText.isNotEmpty && !_textController.isClosed) {
+                  _textController.add(trimmedText);
                 }
               }
             }
@@ -241,8 +242,9 @@ class A2uiAgentConnector {
               if (part is DataPart) {
                 _processA2uiMessages(part.data);
               } else if (part is TextPart) {
-                if (!_textController.isClosed) {
-                  _textController.add(part.text);
+                final String trimmedText = part.text.trim();
+                if (trimmedText.isNotEmpty && !_textController.isClosed) {
+                  _textController.add(trimmedText);
                 }
               }
             }