fix(ai): guard inferred unified weekly usage against monthly probe network errors

- Ensure inferred weekly credits on unified accounts are only used when the monthly probe successfully returns a config without positive monthly quota.
- If the monthly probe fails (transient network error), reject the inferred report so AuthStorage's retain-last-good cache preserves the previous valid snapshot.
- Add unit test verifying network error behavior on unified monthly probe.

packages/ai/src/usage/xai-oauth.ts

@@ -373,10 +373,21 @@ export const xaiOauthUsageProvider: UsageProvider = {
 					: null;
 		}
 
-		// When an account has an explicit monthly included quota and weekly credits
-		// were only inferred from an omitted percentage field, prefer the monthly quota
-		// so pure monthly unified accounts do not render a phantom weekly credit.
-		const effectiveWeekly = weekly && (!monthly || !weekly.inferredPercent) ? weekly : null;
+		// When an account is marked unified billing and weekly credits were only inferred
+		// from an omitted percentage field:
+		// - If a positive monthly quota is returned, use the monthly quota alone.
+		// - If the monthly endpoint returned a valid config without positive monthly quota,
+		//   confirm that this account relies on the weekly reset cycle and use weekly.
+		// - If the monthly fetch failed (transient network error), reject inferred weekly
+		//   so AuthStorage's retain-last-good cache preserves the previous valid snapshot.
+		let effectiveWeekly = weekly;
+		if (weekly?.inferredPercent && creditsLooksUnified) {
+			if (monthly) {
+				effectiveWeekly = null;
+			} else if (!monthlyPayload || !isRecord(monthlyPayload) || !isRecord(monthlyPayload.config)) {
+				effectiveWeekly = null;
+			}
+		}
 		if (!effectiveWeekly && !monthly) return null;
 
 		const limits: UsageLimit[] = [];

packages/ai/test/xai-oauth-usage.test.ts

@@ -125,6 +125,9 @@ function dualBillingFetch(
 			});
 		}
 		const payload = url.includes("format=credits") ? creditsPayload : monthlyPayload;
+		if (payload === null) {
+			return new Response("internal error", { status: 500 });
+		}
 		return new Response(JSON.stringify(payload), {
 			status: 200,
 			headers: { "content-type": "application/json" },
@@ -278,6 +281,31 @@ describe("xai-oauth usage provider", () => {
 		expect(credits?.window?.resetsAt).toBe(Date.parse(periodEnd));
 	});
 
+	it("rejects inferred unified weekly usage when the monthly probe encounters a network error", async () => {
+		const periodEnd = new Date(Date.now() + 6 * 24 * 60 * 60 * 1000).toISOString();
+		const periodStart = new Date(Date.now() - 24 * 60 * 60 * 1000).toISOString();
+		const report = await xaiOauthUsageProvider.fetchUsage(
+			{ provider: "xai-oauth", credential: makeCredential() },
+			{
+				fetch: dualBillingFetch(
+					{
+						config: {
+							currentPeriod: {
+								end: periodEnd,
+								start: periodStart,
+								type: "USAGE_PERIOD_TYPE_WEEKLY",
+							},
+							isUnifiedBillingUser: true,
+						},
+					},
+					null,
+				).fetch,
+			},
+		);
+
+		expect(report).toBeNull();
+	});
+
 	it("falls back to monthly included quota when credits has no percent fields", async () => {
 		const { fetch, calls } = dualBillingFetch(makeUnifiedCreditsPayload(), makeUnifiedMonthlyPayload());
 		const report = await xaiOauthUsageProvider.fetchUsage(