M1.9: the MCP connect-failure test holds its own closed port

remote_connect_fails_fast bound a port and dropped it, so another test or
process could take the port before the connect and make it succeed. It now
holds a socket bound without listening (socket2, a test-only dependency):
connections to it are refused and nothing else can bind it meanwhile. It
also checks the failure comes within 5 s.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Cargo.lock

@@ -3549,6 +3549,16 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "socket2"
+version = "0.6.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c3d1e2c7f27f8d4cb10542a02c49005dbd6e93095799d6f3be745fae9f8fedd4"
+dependencies = [
+ "libc",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "spirv"
 version = "0.4.0+sdk-1.4.341.0"
@@ -4194,6 +4204,7 @@ dependencies = [
  "log",
  "proptest",
  "serde_json",
+ "socket2",
  "vectorcraft-doc",
  "vectorcraft-engine",
  "vectorcraft-format",

Cargo.toml

@@ -38,6 +38,8 @@ serde = { version = "1", features = ["derive", "rc"] }
 serde_json = { version = "1", features = ["float_roundtrip"] }
 log = "0.4"
 proptest = "1"
+# tests: a TCP port bound without listening (connections to it are refused)
+socket2 = "0.6"
 
 # rendering / formats
 vello_cpu = "0.2"

crates/mcp/Cargo.toml

@@ -23,3 +23,4 @@ log = { workspace = true }
 vectorcraft-testkit = { workspace = true }
 proptest = { workspace = true }
 image = { workspace = true }
+socket2 = { workspace = true }

crates/mcp/src/tests.rs

@@ -377,9 +377,17 @@ fn remote_forwards_methods() {
 
 #[test]
 fn remote_connect_fails_fast() {
-    // Bind then drop to get a port that's (almost certainly) closed.
-    let port = std::net::TcpListener::bind("127.0.0.1:0").unwrap().local_addr().unwrap().port();
+    // A port this test holds, bound but not listening, so connecting to it is refused. (A port
+    // bound and dropped could be taken by another process or test before the connect.)
+    let held = socket2::Socket::new(socket2::Domain::IPV4, socket2::Type::STREAM, None).unwrap();
+    held.bind(&std::net::SocketAddr::from(([127, 0, 0, 1], 0)).into()).unwrap();
+    let port = held.local_addr().unwrap().as_socket().unwrap().port();
+    let start = std::time::Instant::now();
     assert!(Remote::connect(&format!("127.0.0.1:{port}")).is_err());
+    assert!(start.elapsed() < std::time::Duration::from_secs(5), "took {:?}", start.elapsed());
+    // Nothing else could listen there meanwhile.
+    assert!(std::net::TcpListener::bind(format!("127.0.0.1:{port}")).is_err());
+    drop(held);
 }
 
 #[test]