fix(infra): add NaN guard for unparseable timestamp in session-cost-usage (#99420)

new Date(parsed.timestamp).getTime() can return NaN for unparseable
strings. Without a guard, NaN silently propagates into downstream
usage/cost calculations and corrupts billing data.

Add Number.isNaN(timestamp) check, falling back to 0 (same default
as the path when no timestamp key is present).

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

src/infra/session-cost-usage.ts

@@ -2743,6 +2743,9 @@ export async function loadSessionLogs(params: {
       let timestamp = 0;
       if (typeof parsed.timestamp === "string") {
         timestamp = new Date(parsed.timestamp).getTime();
+        if (Number.isNaN(timestamp)) {
+          timestamp = 0;
+        }
       } else if (typeof message.timestamp === "number") {
         timestamp = message.timestamp;
       }