brokerd: close the connection after the final frame again

hold_open (d21baa2) kept each connection open for up to two seconds
after its final frame, reading and dropping anything the peer sent, to
hide a test client that set a read timeout after the handler had closed.
On macOS that call fails with EINVAL; the clients now allow for it
(00a85c1, d7009dc), so the handler goes back to closing at once.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/brokerd/src/broker.rs

@@ -25,10 +25,6 @@ use crate::runner::Runtime;
 /// The failure message sent when a peer leaves at the last look, after the table entry is taken.
 pub const GONE: &str = "the requester went away";
 
-/// How long to keep a socket open after the final frame, so the peer can finish reading before its
-/// end of the pair is half-closed.
-const HOLD_OPEN: Duration = Duration::new(2, 0);
-
 /// A line printer: one line per call, owned by the broker.
 pub type Log = Box<dyn Fn(&str) + Send + Sync>;
 
@@ -189,23 +185,6 @@ pub fn alive(stream: &UnixStream) -> bool {
     }
 }
 
-/// Keep the socket open after the final frame, until the peer closes it or the timeout elapses.
-/// Dropping the peer's end makes its next `set_read_timeout` fail on macOS, so we hold the read
-/// half open for as long as the peer might still be reading.
-pub(crate) fn hold_open(stream: &mut UnixStream) {
-    if stream.set_read_timeout(Some(HOLD_OPEN)).is_err() {
-        return;
-    }
-    let mut byte = [0u8; 1];
-    loop {
-        match stream.read(&mut byte) {
-            Ok(0) => break, // the peer closed
-            Ok(_) => continue,
-            Err(_) => break,
-        }
-    }
-}
-
 /// Answer one call: decide, and either answer it or wait for an approval.
 pub fn handle(mut stream: UnixStream, broker: &Broker) {
     let Some(envelope) = read_request(&mut stream) else {
@@ -215,7 +194,6 @@ pub fn handle(mut stream: UnixStream, broker: &Broker) {
     let msg = envelope.msg;
     let Message::ToolRequest(request) = msg else {
         forbid(broker, &mut stream, id, &msg, "broker.sock"); // 2. not a tool request
-        hold_open(&mut stream);
         return;
     };
 
@@ -230,7 +208,6 @@ pub fn handle(mut stream: UnixStream, broker: &Broker) {
     };
     if let Some(answer) = answer {
         let _ = send(&mut stream, id, true, Message::ToolResponse(answer)); // 4. the final frame
-        hold_open(&mut stream);
     }
 }