fastmcp: allow passing Tool directly to .add_tool

[vbarda]

This change allows passing the FastMCP Tool instance directly via a new fastmcp_server.add_tool_instance method.

This is useful if someone needs to construct the Tool instance manually, without inspecting a function. For example, this can be used to convert LangChain tools to FastMCP-compatible tools and add those to an existing FastMCP server.

[dsp-ant]

I am okay with the general idea of allowing the registration of a Tool instance directly. However, I personally prefer using a separate function name over an overload, since the overload leads to add_tool having a very funky type signature. I personally would prefer if we add a add_tool_instance function instead. Would love to hear @Kludex or @ihrpr's opinion on the matter.

[vbarda]

I am okay with the general idea of allowing the registration of a Tool instance directly. However, I personally prefer using a separate function name over an overload, since the overload leads to add_tool having a very funky type signature. I personally would prefer if we add a add_tool_instance function instead. Would love to hear @Kludex or @ihrpr's opinion on the matter.

sounds good - I will add a separate method!