chore: spawn linked async req process in http1 pool

I wanted to remove the process monitoring, but without the async request
actively monitoring the calling process, the request wasn't being
short-circuited when the caller exited. I'm not entirely sure why.

lib/finch/http1/pool.ex

@@ -79,7 +79,7 @@ defmodule Finch.HTTP1.Pool do
     owner = self()
 
     pid =
-      spawn(fn ->
+      spawn_link(fn ->
         monitor = Process.monitor(owner)
         request_ref = receive_next_within!(10)
 
@@ -120,6 +120,7 @@ defmodule Finch.HTTP1.Pool do
 
   @impl Finch.Pool
   def cancel_async_request({_, _, _, pid}) do
+    Process.unlink(pid)
     Process.exit(pid, :shutdown)
     :ok
   end

test/finch/http1/pool_test.exs

@@ -113,7 +113,7 @@ defmodule Finch.HTTP1.PoolTest do
 
       assert_receive {:req_pid, pid}
 
-      Process.exit(caller, :kill)
+      Process.exit(caller, :shutdown)
       Process.sleep(200)
 
       refute Process.alive?(pid)