fix(e2e): use process groups for reliable SIGINT delivery

Instead of using 'exec' which breaks shell syntax like environment
variable assignments, create a new process group on Unix platforms.
This allows send_ctrlc to deliver SIGINT to the entire process tree
including sh -> vite -> node chains.

The previous 'exec' approach failed for commands like:
  FOO=1 vite run hello
because exec can't handle shell variable assignment syntax.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: branchseer

crates/vite_task_bin/tests/e2e_snapshots/main.rs

@@ -175,20 +175,21 @@ async fn run_case_inner(tmpdir: &AbsolutePath, fixture_path: &Path, fixture_name
         let mut e2e_outputs = String::new();
         for step in e2e.steps {
             let mut cmd = Command::new(&shell_exe);
-            // On Unix, use exec to replace the shell with the command, reducing process hierarchy
-            // This makes Ctrl+C signal propagation more reliable
-            #[cfg(unix)]
-            let cmd_str = format!("exec {}", step.cmd());
-            #[cfg(not(unix))]
-            let cmd_str = step.cmd().to_string();
-
             cmd.arg("-c")
-                .arg(cmd_str)
+                .arg(step.cmd())
                 .env_clear()
                 .env("PATH", &e2e_env_path)
                 .env("NO_COLOR", "1")
                 .current_dir(e2e_stage_path.join(&e2e.cwd));
 
+            // Create a new process group on Unix so we can send signals to the entire group
+            #[cfg(unix)]
+            {
+                #[allow(unused_imports)]
+                use std::os::unix::process::CommandExt;
+                cmd.process_group(0);
+            }
+
             // On Windows, inherit PATHEXT for executable lookup
             if cfg!(windows) {
                 if let Ok(pathext) = std::env::var("PATHEXT") {