Add serialization test for Annotations

Author: ptitjes

kotlin-sdk-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/TypesTest.kt

@@ -6,6 +6,8 @@ import kotlin.test.assertEquals
 import kotlin.test.assertIs
 import kotlin.test.assertNotEquals
 import kotlin.test.assertTrue
+import kotlin.time.ExperimentalTime
+import kotlin.time.Instant
 
 class TypesTest {
 
@@ -236,6 +238,23 @@ class TypesTest {
         assertEquals("text/x-rust", textResource.mimeType)
     }
 
+    @OptIn(ExperimentalTime::class)
+    @Test
+    fun `should serialize and deserialize annotations correctly`() {
+        val annotations = Annotations(
+            audience = listOf(Role.assistant),
+            lastModified = Instant.parse("2025-06-18T00:00:00Z"),
+            priority = 0.5,
+        )
+
+        val json = McpJson.encodeToString(annotations)
+        val decoded = McpJson.decodeFromString<Annotations>(json)
+
+        assertEquals(listOf(Role.assistant), decoded.audience)
+        assertEquals(Instant.parse("2025-06-18T00:00:00Z"), decoded.lastModified)
+        assertEquals(0.5, decoded.priority)
+    }
+
     @Test
     fun `should serialize and deserialize prompt message correctly`() {
         val imageContent = ImageContent(