fix: handle null AiMessage.text() to prevent NPE

contrib/langchain4j/src/main/java/com/google/adk/models/langchain4j/LangChain4j.java

@@ -607,8 +607,11 @@ private List<Part> toParts(AiMessage aiMessage) {
               });
       return parts;
     } else {
-      Part part = Part.builder().text(aiMessage.text()).build();
-      return List.of(part);
+      String text = aiMessage.text();
+      if (text == null) {
+        return List.of();
+      }
+      return List.of(Part.builder().text(text).build());
     }
   }
 

contrib/langchain4j/src/test/java/com/google/adk/models/langchain4j/LangChain4jTest.java

@@ -970,4 +970,27 @@ void testNoUsageMetadataWithoutEstimator() {
     // IMPORTANT: usageMetadata should be empty when no TokenUsage and no estimator
     assertThat(response.usageMetadata()).isEmpty();
   }
+
+  @Test
+  @DisplayName("Should handle null AiMessage text without throwing NPE")
+  void testGenerateContentWithNullAiMessageText() {
+    // Given
+    final LlmRequest llmRequest =
+        LlmRequest.builder().contents(List.of(Content.fromParts(Part.fromText("Hello")))).build();
+
+    final ChatResponse chatResponse = mock(ChatResponse.class);
+    final AiMessage aiMessage = mock(AiMessage.class);
+    when(aiMessage.text()).thenReturn(null);
+    when(aiMessage.hasToolExecutionRequests()).thenReturn(false);
+    when(chatResponse.aiMessage()).thenReturn(aiMessage);
+    when(chatModel.chat(any(ChatRequest.class))).thenReturn(chatResponse);
+
+    // When
+    final Flowable<LlmResponse> responseFlowable = langChain4j.generateContent(llmRequest, false);
+    final LlmResponse response = responseFlowable.blockingFirst();
+    // Then - no NPE thrown, and content has no text parts
+    assertThat(response).isNotNull();
+    assertThat(response.content()).isPresent();
+    assertThat(response.content().get().parts().orElse(List.of())).isEmpty();
+  }
 }