fix(client) add encodeDefault for field with non spec default value (modelcontextprotocol#99)

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

* add test

Author: shiqicao

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

@@ -1054,6 +1054,8 @@ public data class Tool(
         val properties: JsonObject = EmptyJsonObject,
         val required: List<String>? = null,
     ) {
+        @OptIn(ExperimentalSerializationApi::class)
+        @EncodeDefault
         val type: String = "object"
     }
 }

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 default value`() {
+        val json = Json(from = McpJson) {
+            encodeDefaults = false
+        }
+        json.encodeToString(getWeatherTool) shouldEqualJson getWeatherToolJson
+    }
 }