Kill background jobs on interrupt (#16285)

# Description
This PR kills all background jobs on interrupt, as a fix for
https://github.com/nushell/nushell/issues/15947.

# User-Facing Changes
If you run the following: `job spawn { print "job spawned"; ^sleep
infinity }; ^sleep infinity`, then hit ctrl-c, the current behavior is
that the `sleep` process from the job will not be killed, it will
reparented to init. With this change, the process will be killed on
ctrl-c.

# Tests + Formatting
I was unsure of the best way to write a test for this.

# After Submitting

---------

Co-authored-by: 132ikl <132@ikl.sh>

crates/nu-protocol/src/pipeline/handlers.rs

@@ -68,6 +68,19 @@ impl Handlers {
         })
     }
 
+    /// Registers a new handler which persists for the entire process lifetime.
+    ///
+    /// Only use this for handlers which should exist for the lifetime of the program.
+    /// You should prefer to use `register` with a `HandlerGuard` when possible.
+    pub fn register_unguarded(&self, handler: Handler) -> Result<(), ShellError> {
+        let id = self.next_id.next()?;
+        if let Ok(mut handlers) = self.handlers.lock() {
+            handlers.push((id, handler));
+        }
+
+        Ok(())
+    }
+
     /// Runs all registered handlers.
     pub fn run(&self, action: SignalAction) {
         if let Ok(handlers) = self.handlers.lock() {

src/signals.rs

@@ -9,6 +9,21 @@ pub(crate) fn ctrlc_protection(engine_state: &mut EngineState) {
     engine_state.set_signals(Signals::new(interrupt.clone()));
 
     let signal_handlers = Handlers::new();
+
+    // Register a handler to kill all background jobs on interrupt.
+    signal_handlers
+        .register_unguarded({
+            let jobs = engine_state.jobs.clone();
+            Box::new(move |action| {
+                if action == SignalAction::Interrupt {
+                    if let Ok(mut jobs) = jobs.lock() {
+                        let _ = jobs.kill_all();
+                    }
+                }
+            })
+        })
+        .expect("Failed to register interrupt signal handler");
+
     engine_state.signal_handlers = Some(signal_handlers.clone());
 
     ctrlc::set_handler(move || {