fix(canvas): guard undefined lastErrorRate and period dates in metrics (PR #1250)

- DetailsTab: use `(data.lastErrorRate ?? 0)` instead of bare multiply to
  prevent NaN% when the field is absent on pre-provisioning workspaces.
- WorkspaceUsage: make formatPeriod accept optional start/end strings;
  return "—" for undefined so the usage period shows blank rather than
  "Invalid Date" for provisioning/partial workspaces.

Refs: #1139

Co-authored-by: Molecule AI Fullstack (floater) <fullstack-floater@agents.moleculesai.app>

canvas/src/components/WorkspaceUsage.tsx

@@ -98,7 +98,8 @@ export function WorkspaceUsage({ workspaceId }: WorkspaceUsageProps) {
   );
 }
 
-function formatPeriod(start: string, end: string): string {
+function formatPeriod(start: string | undefined, end: string | undefined): string {
+  if (!start || !end) return "—";
   const fmt = (s: string) =>
     new Date(s).toLocaleDateString(undefined, {
       month: "short",

canvas/src/components/tabs/DetailsTab.tsx

@@ -189,7 +189,7 @@ export function DetailsTab({ workspaceId, data }: Props) {
             <Row label="Parent" value={data.parentId || "root"} mono />
             <Row label="Active Tasks" value={String(data.activeTasks)} />
             {data.status === "degraded" && (
-              <Row label="Error Rate" value={`${(data.lastErrorRate * 100).toFixed(0)}%`} />
+              <Row label="Error Rate" value={`${((data.lastErrorRate ?? 0) * 100).toFixed(0)}%`} />
             )}
             {isRestartable && (
               <div className="pt-2">