Windows build broken by #[cfg] on tokio::select! branch (#9404)

#9400 (WIN-2003) added the ctrl_break() handler as a `#[cfg(windows)]`
branch inside the two Windows-path `tokio::select!` blocks in
shutdown_signal. tokio's `select!` macro does not accept `#[cfg(...)]`
attributes on individual branches, so windmill-common fails to compile
on Windows ("no rules expected this token in macro call").

This slipped through CI because the only job that builds the backend on
Windows is cli-tests.yml's `test-windows`, which triggers only on
`cli/**` changes — #9400 was backend-only.

Fix: define `ctrl_break()` for the whole `not(any(linux, macos))` scope
instead of just `windows`. On Windows it awaits the real CTRL_BREAK
signal; on other non-unix targets it is a never-resolving future, so the
branch is inert there. The select! branches become plain (no per-branch
`#[cfg]`), which the macro accepts.

Verified: the `#[cfg]`-on-branch form reproduces the exact macro error
against tokio 1.46.1, and the fixed form compiles clean.

Fixes WIN-2003 (Windows build regression)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/windmill-common/src/lib.rs

@@ -282,10 +282,22 @@ pub async fn shutdown_signal(
         Ok(())
     }
 
-    #[cfg(windows)]
+    // Defined for the whole non-unix scope (not just windows) so it can be a
+    // plain `tokio::select!` branch: that macro does not accept `#[cfg(...)]`
+    // attributes on individual branches. On non-windows non-unix targets the
+    // future never resolves, so the branch is effectively inert there.
+    #[cfg(not(any(target_os = "linux", target_os = "macos")))]
     async fn ctrl_break() -> std::io::Result<()> {
-        tokio::signal::windows::ctrl_break()?.recv().await;
-        Ok(())
+        #[cfg(windows)]
+        {
+            tokio::signal::windows::ctrl_break()?.recv().await;
+            Ok(())
+        }
+        #[cfg(not(windows))]
+        {
+            std::future::pending::<()>().await;
+            Ok(())
+        }
     }
 
     #[cfg(any(target_os = "linux", target_os = "macos"))]
@@ -306,7 +318,6 @@ pub async fn shutdown_signal(
         _ = tokio::signal::ctrl_c() => {
             tracing::info!("shutdown monitor received ctrl-c");
         },
-        #[cfg(windows)]
         _ = ctrl_break() => {
             tracing::info!("shutdown monitor received ctrl-break");
         },
@@ -331,7 +342,6 @@ pub async fn shutdown_signal(
             _ = tokio::signal::ctrl_c() => {
                 tracing::error!("2nd shutdown monitor received ctrl-c")
             },
-            #[cfg(windows)]
             _ = ctrl_break() => {
                 tracing::error!("2nd shutdown monitor received ctrl-break")
             },