feat: Add output schema support to MCP tools

[bigdevlarry]

The changes add comprehensive support for outputSchema in the Tools definition, enabling MCP servers to specify the expected structure and types of tool responses.

This includes:

Added outputSchema parameter to Builder::addTool() method
Updated the Tool class constructor and JSON serialization to support outputSchema
Added validation for outputSchema structure and type requirements
Enhanced existing tests to verify outputSchema functionality works correctly
Fixed related type annotation issues and code style problems

Motivation and Context

How Has This Been Tested?

Breaking Changes

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
• [x ] I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[bigdevlarry]

Hi @chr-hertel, & @CodeWithKyrian the OutputSchema support PR is back up

[bigdevlarry]

Hi @chr-hertel, @Nyholm can I get some eyes on here for the output schema support. Ty

[Nyholm]

Interesting. I do like the idea. I'll spend some more time to read the code.

[chr-hertel]

In general this looks pretty good already, just one thing i noticed with one test scenario. structuredContent should be optional in response payload

[chr-hertel]

I wonder a bit about this change here - it's not adding value to me. spec is unclear about it, but i think we shouldn't just add it always.

[bigdevlarry]

@chr-hertel this has now been actioned, so structrued content will only be added if outputschema is present and explicility specified

[bigdevlarry]

@chr-hertel To make structuredContent optional in the response payload, I'll have to change the behavior so that outputSchema is only present when explicitly provided in the #[McpTool] attribute, not auto-generated from return types or DocBlocks. When an outputSchema is explicitly provided, structuredContent must be included in the response (as required by the MCP spec). When no outputSchema is provided, neither outputSchema nor structuredContent is included.

[chr-hertel]

The case that i was pointing out was mostly just returning a string, but still had that result => '...' structure on top - i just can't find any reference for that in the spec or do i miss sth?

[bigdevlarry]

The case that i was pointing out was mostly just returning a string, but still had that result => '...' structure on top - i just can't find any reference for that in the spec or do i miss sth?

Good shout ! I was working with the wrong assumption that the result wrapper was in the spec. structuredContent now matches the outputSchema properties directly, no more top level result wrapping. The structure is as in the MCP spec example. I've updated the diagram on the PR to reflect the test as well