fix(client) add encodeDefault for field with non spec default value

Author: shiqicao

src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types.kt

@@ -3,6 +3,8 @@
 package io.modelcontextprotocol.kotlin.sdk
 
 import io.modelcontextprotocol.kotlin.sdk.shared.McpJson
+import kotlinx.serialization.EncodeDefault
+import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonObject
@@ -1050,6 +1052,8 @@ public data class Tool(
         val properties: JsonObject = EmptyJsonObject,
         val required: List<String>? = null,
     ) {
+        @OptIn(ExperimentalSerializationApi::class)
+        @EncodeDefault
         val type: String = "object"
     }
 }