feat: client-side support for SEP-1577 sampling with tools

[jlowin]

SEP-1577 added tool calling support to sampling (via #1594). This PR adds the necessary client-side support:

1. Capability advertisement: The spec requires servers to error when tools or toolChoice are provided but clientCapabilities.sampling.tools is missing. The SDK's types already support this, but ClientSession had no way to advertise it—servers would always see the capability as missing and error. This adds a sampling_capabilities parameter. If not provided, the previous logic applies: the presence of a sampling_callback enables basic sampling support without tools.

session = ClientSession(
    read_stream, write_stream,
    sampling_callback=my_callback,
    sampling_capabilities=SamplingCapability(
        tools=SamplingToolsCapability()
    ),
)

2. Return type for tool responses: When tools are provided in a sampling request, the LLM may respond with tool calls, requiring CreateMessageResultWithTools (which supports array content). The SamplingFnT callback and ClientResultType didn't include this type, so the client's response validation would reject it before it could be sent back to the server. This adds CreateMessageResultWithTools to both.

FWIW, with these changes we have full agentic sampling wired up in a branch of FastMCP, which is extremely exciting!

[maxisbey]

thoughts on moving to the bottom of kwargs or keyword only to avoid breaking positional uses of the constructor? I'm probably leaning towards positional keyword only

[jlowin]

Good call! (assuming you meant leaning "keyword" only, and I agree - will update)

[maxisbey]

woops yea meant keyword only haha