Add support for _meta property on Tool definition

[tuntisz]

Is your feature request related to a problem? Please describe.

Currently, the ruby-sdk does not support the _meta property for tool definitions, which is part of the MCP specification. The TypeScript SDK and other official SDKs include support for _meta fields, allowing clients and servers to attach additional metadata to their interactions. Without this support in the Ruby SDK, Ruby-based MCP servers cannot provide or consume metadata that may be critical for certain protocol extensions or implementation-specific features.

Describe the solution you'd like

Add support for the _meta property in tool definitions within the ruby-sdk, following the MCP specification guidelines:

1. Update the MCP::Tool class to accept an optional _meta parameter in both class-based and define method approaches
2. Include _meta in the tool schema serialization when responding to tools/list requests
3. Validate _meta keys according to the specification format rules (prefix/name segments)
4. Pass through _meta in tool responses where appropriate

Implementation should follow the pattern used in the TypeScript SDK where _meta is an optional field in tool definitions.

Example usage:

class MyTool < MCP::Tool
  description "Example tool with metadata"
  input_schema(
    properties: { message: { type: "string" } },
    required: ["message"]
  )
  
  # New _meta support
  meta({
    "example.com/priority" => "high",
    "internal-id" => "tool-123"
  })
  
  def self.call(message:, server_context:)
    MCP::Tool::Response.new([{ type: "text", text: "OK" }])
  end
end

# Or with define method
tool = MCP::Tool.define(
  name: "my_tool",
  description: "Example tool",
  _meta: {
    "example.com/priority" => "high"
  }
) do |args, server_context|
  MCP::Tool::Response.new([{ type: "text", text: "OK" }])
end

Describe alternatives you've considered

1. Workaround via custom annotations: While the SDK supports custom annotations, these don't follow the MCP specification for _meta and aren't interoperable with other MCP implementations.

2. Fork and patch: Creating a custom fork of the ruby-sdk with _meta support, but this creates maintenance burden and diverges from the official SDK.

3. Wait for automatic generation: Since the TypeScript SDK already supports _meta, waiting for this to be ported might be an option, but proactive implementation would benefit Ruby users sooner.

Additional context

The _meta property is defined in the MCP specification ([source](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/docs/specification/draft/basic/index.mdx#general-fields)) as a reserved property for attaching metadata. The TypeScript SDK implementation in types.ts includes _meta as an optional field in tool definitions.

Key specification requirements for _meta:

• Keys must follow the format: optional prefix (with /) + name
• Prefixes use dot-separated labels ending with /
• The modelcontextprotocol.* and mcp.* prefixes are reserved
• Names must begin/end with alphanumeric characters

This feature would bring the ruby-sdk to feature parity with other official SDKs and enable more sophisticated MCP implementations in Ruby.