Support native MCP Tool result types (in the extensions)

[eiriktsarpalis]

Originally posted by @twenzel in PederHP/mcpdotnet#55.

See
https://github.com/PederHP/mcpdotnet/pull/28/files/459abaf22645853a51dd15dedcd8b6e439554fc7#r1986110059

[stephentoub]

We should be able to implement this easily once dotnet/extensions#6175 lands in a new build of Microsoft.Extensions.AI that's consumed here.