[MCP SDK] ToolCallHandler now returns structuredContent if applicable

[xavierleune]

Q	A
Bug fix?	no
New feature?	yes
Docs?	no
Issues	Fix #...
License	MIT

As per documentation we should include any structured content in a dedicated field

Structured content is returned as a JSON object in the structuredContent field of a result.
For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.

[JoshuaBehrens]

I have a similar patch in local testing :)

[OskarStark]

Please rebase, thanks

[xavierleune]

@OskarStark done 👍

[chr-hertel]

Two things, I think this is good timing to add a test for checking the two options for type text.

Isn't it that with the current approach we end up with something like "text": "\"foobar\"" for simple, unstructured text values?