fix(unix): ignore SIGPIPE in native Flutter entry (#16322)

* fix(linux): ignore SIGPIPE in Flutter runner

Initialize SIGPIPE before entering the Rust core so writes to closed IPC peers return EPIPE instead of terminating the server. Add a native-entry regression test for the failure reported in #16297.

* test(linux): remove SIGPIPE runner test

* fix(flutter): initialize SIGPIPE in the Rust native entry

Share SIGPIPE initialization between the Linux and macOS native runners before core startup. Remove the Linux C++ initialization.

* fix: share SIGPIPE initialization with Sciter startup

Initialize SIGPIPE before global_init in core_main, shared by Flutter and Sciter on Linux and macOS.

* fix(flutter): keep SIGPIPE setup in the native FFI entry

Sciter already receives SIGPIPE initialization from the Rust runtime. Scope the missing startup initialization to native Flutter runners on Linux and macOS.

src/flutter.rs

@@ -105,6 +105,16 @@ fn load_plugin_in_app_path(dll_name: &str) -> Result<Library, LibError> {
 #[cfg(not(windows))]
 #[no_mangle]
 pub extern "C" fn rustdesk_core_main() -> bool {
+    #[cfg(any(target_os = "linux", target_os = "macos"))]
+    {
+        use hbb_common::libc;
+
+        // Native runners bypass Rust's startup, which normally ignores SIGPIPE.
+        if unsafe { libc::signal(libc::SIGPIPE, libc::SIG_IGN) } == libc::SIG_ERR {
+            eprintln!("Failed to ignore SIGPIPE: {}", std::io::Error::last_os_error());
+            std::process::exit(1);
+        }
+    }
     #[cfg(not(any(target_os = "android", target_os = "ios")))]
     if crate::core_main::core_main().is_some() {
         return true;