Add getNativeClient API to VectorStore interface

Adds getNativeClient API to VectorStore interface allowing access to the underlying native client implementation.

This change:
- Adds getNativeClient() default method to VectorStore interface returning Optional<T>
- Implements getNativeClient() in all vector store implementations exposing their respective native clients
- Adds integration tests verifying native client access for all implementations

Fixes: #2137

Signed-off-by: Soby Chacko <soby.chacko@broadcom.com>

vector-stores/spring-ai-pgvector-store/src/main/java/org/springframework/ai/vectorstore/pgvector/PgVectorStore.java

@@ -476,6 +476,13 @@ public class PgVectorStore extends AbstractObservationVectorStore implements Ini
 		return SIMILARITY_TYPE_MAPPING.get(this.distanceType).value();
 	}
 
+	@Override
+	public <T> Optional<T> getNativeClient() {
+		@SuppressWarnings("unchecked")
+		T client = (T) this.jdbcTemplate;
+		return Optional.of(client);
+	}
+
 	/**
 	 * By default, pgvector performs exact nearest neighbor search, which provides perfect
 	 * recall. You can add an index to use approximate nearest neighbor search, which

vector-stores/spring-ai-pgvector-store/src/test/java/org/springframework/ai/vectorstore/pgvector/PgVectorStoreIT.java

@@ -482,6 +482,15 @@ public class PgVectorStoreIT {
 		});
 	}
 
+	@Test
+	void getNativeClientTest() {
+		this.contextRunner.run(context -> {
+			PgVectorStore vectorStore = context.getBean(PgVectorStore.class);
+			Optional<JdbcTemplate> nativeClient = vectorStore.getNativeClient();
+			assertThat(nativeClient).isPresent();
+		});
+	}
+
 	@SpringBootConfiguration
 	@EnableAutoConfiguration(exclude = { DataSourceAutoConfiguration.class })
 	public static class TestApplication {