Enable dynamic filter expressions for QuestionAnswerAdvisor

- Add FILTER_EXPRESSION advisor context parameter to update filter expressions per call/stream
 - Implement dynamic filter expression handling in QuestionAnswerAdvisor
 - Add unit tests for dynamic filter expression functionality
 - Update documentation with usage examples

 Resolves #887

spring-ai-core/src/test/java/org/springframework/ai/chat/client/QuestionAnswerAdvisorTests.java

@@ -0,0 +1,114 @@
+/*
+ * Copyright 2024-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.ai.chat.client;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.when;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Captor;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.ai.chat.client.advisor.QuestionAnswerAdvisor;
+import org.springframework.ai.chat.messages.Message;
+import org.springframework.ai.chat.messages.MessageType;
+import org.springframework.ai.chat.model.ChatModel;
+import org.springframework.ai.chat.model.ChatResponse;
+import org.springframework.ai.chat.model.Generation;
+import org.springframework.ai.chat.prompt.Prompt;
+import org.springframework.ai.document.Document;
+import org.springframework.ai.vectorstore.SearchRequest;
+import org.springframework.ai.vectorstore.VectorStore;
+import org.springframework.ai.vectorstore.filter.FilterExpressionBuilder;
+
+/**
+ * @author Christian Tzolov
+ */
+@ExtendWith(MockitoExtension.class)
+public class QuestionAnswerAdvisorTests {
+
+	@Mock
+	ChatModel chatModel;
+
+	@Captor
+	ArgumentCaptor<Prompt> promptCaptor;
+
+	@Captor
+	ArgumentCaptor<SearchRequest> vectorSearchCaptor;
+
+	@Mock
+	VectorStore vectorStore;
+
+	@Test
+	public void qaAdvisorWithDynamicFilterExpressions() {
+
+		when(chatModel.call(promptCaptor.capture()))
+			.thenReturn(new ChatResponse(List.of(new Generation("Your answer is ZXY"))));
+
+		when(vectorStore.similaritySearch(vectorSearchCaptor.capture()))
+			.thenReturn(List.of(new Document("doc1"), new Document("doc2")));
+
+		var qaAdvisor = new QuestionAnswerAdvisor(vectorStore,
+				SearchRequest.defaults().withSimilarityThreshold(0.99d).withTopK(6));
+
+		var chatClient = ChatClient.builder(chatModel)
+			.defaultSystem("Default system text.")
+			.defaultAdvisors(qaAdvisor)
+			.build();
+
+		// @formatter:off
+		var content = chatClient.prompt()
+			.user("Please answer my question XYZ")
+			.advisors(a -> a.param(QuestionAnswerAdvisor.FILTER_EXRESSION, "type == 'Spring'"))
+			.call()
+			.content();
+		//formatter:on
+
+		assertThat(content).isEqualTo("Your answer is ZXY");
+
+		Message systemMessage = promptCaptor.getValue().getInstructions().get(0);
+
+		System.out.println(systemMessage.getContent());
+
+		assertThat(systemMessage.getContent()).isEqualToIgnoringWhitespace("""
+				Default system text.
+				""");
+		assertThat(systemMessage.getMessageType()).isEqualTo(MessageType.SYSTEM);
+
+		Message userMessage = promptCaptor.getValue().getInstructions().get(1);
+
+		assertThat(userMessage.getContent()).isEqualToIgnoringWhitespace("""
+			Please answer my question XYZ
+			Context information is below.
+			---------------------
+			doc1
+			doc2
+			---------------------
+			Given the context and provided history information and not prior knowledge,
+			reply to the user comment. If the answer is not in the context, inform
+			the user that you can't answer the question.
+			""");
+
+		assertThat(vectorSearchCaptor.getValue().getFilterExpression()).isEqualTo(new FilterExpressionBuilder().eq("type", "Spring").build());
+		assertThat(vectorSearchCaptor.getValue().getSimilarityThreshold()).isEqualTo(0.99d);
+		assertThat(vectorSearchCaptor.getValue().getTopK()).isEqualTo(6);
+	}
+}