test: reproduce #270 (solvecraft-cli mcp --connect without a listening app gives no hint)

apps/solvecraft-cli/tests/mcp_connect.rs

@@ -0,0 +1,18 @@
+//! `mcp --connect` to a port where no app listens tells the user how to start one (#270).
+
+use std::net::TcpListener;
+use std::process::{Command, Stdio};
+
+#[test]
+fn mcp_connect_without_a_listening_app_names_the_control_flag() {
+    // A port that was free a moment ago: nothing listens on it.
+    let port = TcpListener::bind("127.0.0.1:0").unwrap().local_addr().unwrap().port();
+    let output = Command::new(env!("CARGO_BIN_EXE_solvecraft-cli"))
+        .args(["mcp", "--connect", &format!("127.0.0.1:{port}")])
+        .stdin(Stdio::null())
+        .output()
+        .unwrap();
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(!output.status.success(), "{stderr}");
+    assert!(stderr.contains("--control"), "say how to start the app with a control port: {stderr}");
+}