add test

Author: shiqicao

src/commonTest/kotlin/ToolSerializationTest.kt

@@ -3,6 +3,7 @@ package io.modelcontextprotocol.kotlin.sdk
 import io.kotest.assertions.json.shouldEqualJson
 import io.modelcontextprotocol.kotlin.sdk.shared.McpJson
 import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
 import kotlinx.serialization.json.JsonPrimitive
 import kotlinx.serialization.json.buildJsonObject
 import kotlin.test.Test
@@ -54,4 +55,11 @@ class ToolSerializationTest {
         assertEquals(expected = getWeatherTool, actual = tool)
     }
 
-}
+    @Test
+    fun `should always serialize field with default value`() {
+        val json = Json(from = McpJson) {
+            encodeDefaults = false
+        }
+        json.encodeToString(getWeatherTool) shouldEqualJson getWeatherToolJson
+    }
+}