Set embedding field as retrievable in Azure Vector Store

Resolves #1628

In Azure Vector Store, the embedding field needs to be retrievable
for proper functionality. While SearchField class doesn't have a
'retrievable' property, the 'hidden' field serves the same purpose.

The fix sets hidden=false on the embedding field to make it retrievable.

Link: #1628

Author: zhaojy01

vector-stores/spring-ai-azure-store/src/main/java/org/springframework/ai/vectorstore/azure/AzureVectorStore.java

@@ -349,6 +349,7 @@ public void afterPropertiesSet() throws Exception {
 			.setSortable(true));
 		fields.add(new SearchField(EMBEDDING_FIELD_NAME, SearchFieldDataType.collection(SearchFieldDataType.SINGLE))
 			.setSearchable(true)
+			.setHidden(false)
 			.setVectorSearchDimensions(dimensions)
 			// This must match a vector search configuration name.
 			.setVectorSearchProfileName(SPRING_AI_VECTOR_PROFILE));