fix(spawn): prefix unused child_pid parameter with underscore

The child_pid parameter is only used on Unix platforms within a
#[cfg(unix)] block, causing an unused variable warning on other
platforms. Prefix with underscore to indicate intentional unused.

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

Author: branchseer

crates/vite_task/src/session/execute/spawn.rs

@@ -74,11 +74,11 @@ pub struct SpawnTrackResult {
 /// - **Unix**: Manually forwards SIGINT to the child process using `kill()`.
 /// - **Windows**: Ctrl+C is automatically forwarded to child processes in the same
 ///   console group, so no manual forwarding is needed.
-async fn wait_for_ctrl_c(child_pid: Option<u32>) {
+async fn wait_for_ctrl_c(_child_pid: Option<u32>) {
     let _ = tokio::signal::ctrl_c().await;
 
     #[cfg(unix)]
-    if let Some(pid) = child_pid {
+    if let Some(pid) = _child_pid {
         // Forward SIGINT to child process on Unix.
         // The child may handle this signal and exit gracefully.
         let _ = nix::sys::signal::kill(