[tools] add support for structured responses

[honzasterba]

Add new response class to be able to return structuredContent as part of tools response in a backwards compatible way

Motivation and Context

Structured responses are an established part of the spec and the SDK should support it.

How Has This Been Tested?

Add new tested and tested with internal MCP server implementation.

Breaking Changes

None

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

[dieter-medium]

Nice job!

Does this not also require an 'outputSchema'?

[honzasterba]

output schema is optional for structured output, for now I just added support for the response field

[atesgoral]

For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.

Not a MUST, but a SHOULD, but I agree with doing it for backwards compatibility in this gem.

[honzasterba]

separated unrelated change into new PR
improved commit message

[honzasterba]

anything else needed to merge this?

[koic]

According to https://modelcontextprotocol.io/specification/2025-06-18/schema#calltoolresult, it seems preferable to add an optional structured_content keyword argument to the existing MCP::Tool::Response rather than creating a new MCP::Tool::StructuredResponse.

[anandaroop]

Thank you for this PR @honzasterba.

I also require this functionality.

Therefore I have tried another approach in line with @koic's suggestion at #147

[koic]

I'll close this since I think the issue has been resolved in #147. Thank you for raising it!