Add support for _meta field on tool schema

[tuntisz]

Motivation and Context

This PR adds support for the _meta property in tool definitions, bringing the Ruby SDK to feature parity with the TypeScript SDK and other official MCP implementations. Previously, Ruby-based MCP servers couldn't attach metadata to tools, limiting interoperability with protocol extensions that rely on this standard field. Resolves issue #123 .

How Has This Been Tested?

• Added comprehensive unit tests for the metadata functionality in test/mcp/tool_test.rb
• Tests verify metadata can be set via both class-based and define method approaches
• Tests confirm _meta appears correctly in tool serialization for tools/list responses
• Verified metadata is optional and doesn't break existing tool definitions
• Ran the official MCP inspector ensuring that the server returns the expect _meta field. Screenshot below

Breaking Changes

None. The metadata parameter is optional and existing code continues to work unchanged.

Types of changes

• New feature (non-breaking change which adds functionality)

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

The implementation follows the MCP specification exactly:

• Metadata is exposed as _meta in the serialized tool schema (matching the spec)
• The Ruby API uses metadata for consistency with other SDK methods
• Both class-based and define method approaches are supported

Example usage:

# Class-based approach
class MyTool < MCP::Tool
  tool_name "my_tool"
  description "Example tool with metadata"
  metadata({ "example.com/priority" => "high" })
end

# Define method approach
server.define_tool(
  name: "my_tool",
  description: "Example tool",
  metadata: { "internal-id" => "tool-123" }
) do |args|
  MCP::Tool::Response.new([{ type: "text", text: "OK" }])
end

[codenamev]

I really need this! Looking forward to seeing this.

[topherbullock]

can you please update the name to meta to closely match the field's name in the spec?

LGTM otherwise, thanks @tuntisz!