fix(handlers): remove unused os and mdb imports in integration test

Both packages were imported but not referenced in the file.
Go build tag "integration" still compiles them — caught by CI.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace-server/internal/handlers/delegation_executor_integration_test.go

@@ -32,12 +32,10 @@ import (
 	"fmt"
 	"net"
 	"net/http"
-	"os"
 	"sync"
 	"testing"
 	"time"
 
-	mdb "github.com/Molecule-AI/molecule-monorepo/platform/internal/db"
 	"github.com/alicebob/miniredis/v2"
 )