fix(handlers): add missing "errors" import to instructions_test.go

Issue #1081 — tests using errors.New(...) without importing "errors".

Fixes: Molecule-AI/molecule-core#1081

workspace-server/internal/handlers/instructions_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"regexp"