Make agent suspend tests release on the suspend signal

The single- and multi-level suspend/restore tests gated their slow
leaf tool on a manual release channel closed 50ms after cancel().
That sleep was a guess at how long the suspend signal takes to reach
the running sub-agent, so the post-tool turn boundary could observe
the release before cancellation and complete the run instead of
checkpointing, making the assertions timing-dependent.

Expose the per-run suspend signal through SuspendSignalFrom in an
export_test shim and have the leaf tools block on it directly. The
tool now returns only once the graceful-suspend signal has actually
propagated to its agent, so suspension is observed deterministically
without sleeps or release channels.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/agent/run.go

@@ -109,9 +109,12 @@ func withSuspendableToolContext(ctx context.Context, tool Tool) (context.Context
 	}
 
 	execCtx, cancel := context.WithCancelCause(ctx)
-	stop := context.AfterFunc(signal, func() {
-		cancel(context.Cause(signal))
-	})
+	stop := context.AfterFunc(
+		signal,
+		func() {
+			cancel(context.Cause(signal))
+		},
+	)
 
 	return execCtx, func() {
 		stop()