feat: add request cancellation and cleanup

[dsp-ant]

Summary

• Add support for request cancellation via notifications
• Implement periodic cleanup of in-flight requests
• Track request state throughout lifecycle
• Fixes Random error thrown on response #88

Test plan

• Test request cancellation
• Verify cleanup of completed requests
• Test concurrent request handling

Depends on #166

[jspahrsummers]

Why isn't this done on completion or cancellation instead? The idea of using a timer for this seems weird to me, and leaves a window (the length of the interval) of possibly unbounded memory growth.

[jspahrsummers]

https://anyio.readthedocs.io/en/stable/cancellation.html#finalization

[jspahrsummers]

OTOH, this seems excessive to do on every new incoming message. Why can't we tie it to completion and cancellation?

[EduPonz]

This doesn't seem to solve #88 for me. I can reproduce triggering a call tool from claude with a call tool handle that simply waits for 200 s and returns a valid message. Claude receives the response but the server crashes

@server.call_tool()
async def handle_call_tool(
    name: str, arguments: dict | None,
) -> list[types.TextContent | types.ImageContent | types.EmbeddedResource]:
    await asyncio.sleep(200)
    response = {"msg": "success"}
    return [types.TextContent(type="text", text=f"{response}")]