Adds optional _meta field to Tool

[jclyne]

Motivation and Context

The recently released openAI Apps SDK 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.

This also adds a Tool.Builder and Tool.build() builders to help with Tool's large constructor

How Has This Been Tested?

Unit tests for serialization added

Breaking Changes

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• 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

[Copilot]

Pull Request Overview

This PR adds an optional _meta field to the Tool class to align with the OpenAI Apps SDK specification, which includes metadata fields on Tool descriptors. The change maintains backward compatibility by defaulting to an empty JSON object.

• Adds _meta field to Tool data class by implementing the WithMeta interface
• Updates Server.addTool() method to accept an optional _meta parameter
• Adds comprehensive serialization tests for the new field

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types.kt	Adds _meta field to Tool class and implements WithMeta interface
kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/Server.kt	Updates addTool() method to accept optional _meta parameter
kotlin-sdk-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/ToolSerializationTest.kt	Adds test coverage for _meta field serialization

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[kpavlov]

Thank you, Jeff

[jclyne]

Thank you, Jeff

My pleasure @kpavlov, looking forward to this merge

[maeryo]

#289
Please refer to the recently merged PR.