test(azure): add unit tests for mutable metadata parsing and distance

Add `AzureVectorStoreMetadataParsingTests` to verify:
- valid JSON → mutable LinkedHashMap copy
- blank/invalid JSON → empty LinkedHashMap
- can inject `distance` without throwing UnsupportedOperationException

These tests fail on the previous implementation and pass with the fix,
guarding against regressions.

Relates to GH-4117

Signed-off-by: Jinwoo Lee <[email protected]>

Author: jinlee1703

vector-stores/spring-ai-azure-store/src/test/java/org/springframework/ai/vectorstore/azure/AzureVectorStoreMetadataTests.java

@@ -0,0 +1,34 @@
+package org.springframework.ai.vectorstore.azure;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+class AzureVectorStoreMetadataTests {
+
+	@Test
+	void returnsMutableMapForBlankOrNull() {
+		Map<String, Object> m1 = AzureVectorStore.parseMetadataToMutable(null);
+		m1.put("distance", 0.1);
+		assertThat(m1).containsEntry("distance", 0.1);
+
+		Map<String, Object> m2 = AzureVectorStore.parseMetadataToMutable("");
+		m2.put("distance", 0.2);
+		assertThat(m2).containsEntry("distance", 0.2);
+
+		Map<String, Object> m3 = AzureVectorStore.parseMetadataToMutable("   ");
+		m3.put("distance", 0.3);
+		assertThat(m3).containsEntry("distance", 0.3);
+	}
+
+	@Test
+	void wrapsParsedJsonInLinkedHashMapSoItIsMutable() {
+		Map<String, Object> map = AzureVectorStore.parseMetadataToMutable("{\"k\":\"v\"}");
+		assertThat(map).containsEntry("k", "v");
+		map.put("distance", 0.4);
+		assertThat(map).containsEntry("distance", 0.4);
+	}
+
+}