Adds optional _meta field to Tool (#339)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
The recently released openAI [Apps SDK
](https://developers.openai.com/apps-sdk/reference/#_meta-fields-the-client-provides)adds
a '__meta_' field on the Tool Descriptor. This would be in addition to
it being available on Messages. It remains optional and defaults to an
empty json object, following existing conventions.


## How Has This Been Tested?
Unit tests for serialization added

## Breaking Changes
N/A

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [X] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [X] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [X] My code follows the repository's style guidelines
- [X] New and existing tests pass locally
- [X] I have added appropriate error handling
- [X] I have added or updated documentation as needed

## Additional context
https://developers.openai.com/apps-sdk

https://developers.openai.com/apps-sdk/reference/#_meta-fields-the-client-provides

---------

Co-authored-by: Pavel Gorgulov <devcrocod@gmail.com>

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

@@ -1242,7 +1242,12 @@ public data class Tool(
      * Optional additional tool information.
      */
     val annotations: ToolAnnotations?,
-) {
+
+    /**
+     * Optional metadata for the tool.
+     */
+    override val _meta: JsonObject = EmptyJsonObject,
+) : WithMeta {
     @Serializable
     public data class Input(val properties: JsonObject = EmptyJsonObject, val required: List<String>? = null) {
         @OptIn(ExperimentalSerializationApi::class)

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

@@ -75,7 +75,8 @@ class RequestSerializerTest {
                     "inputSchema": {
                         "type": "object",
                         "properties": {}
-                    }
+                    },
+                    "_meta": {}
                 }
             ]
         }"""

kotlin-sdk-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/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.json.Json
+import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.JsonPrimitive
 import kotlinx.serialization.json.buildJsonObject
 import kotlin.test.Test
@@ -44,6 +45,9 @@ class ToolSerializationTest {
               }
             },
             "required": ["temperature", "conditions", "humidity"]
+          },
+          "_meta": {
+            "_for_test_only": true
           }
         }
     """.trimIndent()
@@ -91,6 +95,9 @@ class ToolSerializationTest {
             },
             required = listOf("temperature", "conditions", "humidity"),
         ),
+        _meta = buildJsonObject {
+            put("_for_test_only", JsonPrimitive(true))
+        },
     )
 
     //region Serialize
@@ -411,6 +418,7 @@ class ToolSerializationTest {
         name: String = "get_weather",
         title: String? = null,
         outputSchema: String? = null,
+        @Suppress("LocalVariableName") _meta: String? = null,
     ): String {
         val stringBuilder = StringBuilder()
 
@@ -453,6 +461,14 @@ class ToolSerializationTest {
                 )
         }
 
+        stringBuilder
+            .appendLine(",")
+            .append(
+                """
+                "_meta": ${_meta ?: "{}"}
+                """.trimIndent(),
+            )
+
         stringBuilder
             .appendLine()
             .appendLine("}")
@@ -464,6 +480,7 @@ class ToolSerializationTest {
         name: String = "get_weather",
         title: String? = null,
         outputSchema: Tool.Output? = null,
+        @Suppress("LocalVariableName") _meta: JsonObject? = null,
     ): Tool = Tool(
         name = name,
         title = title,
@@ -482,6 +499,7 @@ class ToolSerializationTest {
             required = listOf("location"),
         ),
         outputSchema = outputSchema,
+        _meta = _meta ?: EmptyJsonObject,
     )
 
     //endregion Private Methods