perf: Don't handle expensive requests for rooms we aren't in

Mostly borrowed from dendrite:

https://github.com/element-hq/dendrite/blob/a042861/federationapi/routing/routing.go#L601

src/api/server/state.rs

@@ -1,7 +1,7 @@
 use std::{borrow::Borrow, iter::once};
 
 use axum::extract::State;
-use conduwuit::{Result, at, err, utils::IterStream};
+use conduwuit::{Err, Result, at, debug_warn, err, utils::IterStream};
 use futures::{FutureExt, StreamExt, TryStreamExt};
 use ruma::{OwnedEventId, api::federation::event::get_room_state};
 
@@ -24,6 +24,19 @@ pub(crate) async fn get_room_state_route(
 	.check()
 	.await?;
 
+	if !services
+		.rooms
+		.state_cache
+		.server_in_room(services.globals.server_name(), &body.room_id)
+		.await
+	{
+		debug_warn!(
+			origin = body.origin().as_str(),
+			"Refusing to serve state for room we aren't participating in"
+		);
+		return Err!(Request(NotFound("This server is not participating in that room.")));
+	}
+
 	let shortstatehash = services
 		.rooms
 		.state_accessor