chore: import upstream snapshot with attribution

tests/unit/error-config.test.ts

@@ -0,0 +1,48 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const {
+  ERROR_TYPES,
+  DEFAULT_ERROR_MESSAGES,
+  BACKOFF_CONFIG,
+  COOLDOWN_MS,
+  calculateBackoffCooldown,
+  findMatchingErrorRule,
+  getDefaultErrorMessage,
+  getErrorInfo,
+  matchErrorRuleByStatus,
+  matchErrorRuleByText,
+} = await import("../../open-sse/config/errorConfig.ts");
+
+test("errorConfig exposes centralized client-facing status metadata", () => {
+  assert.deepEqual(ERROR_TYPES[402], {
+    type: "billing_error",
+    code: "payment_required",
+  });
+  assert.equal(DEFAULT_ERROR_MESSAGES[406], "Model not supported");
+  assert.equal(getDefaultErrorMessage(999), "An error occurred");
+  assert.deepEqual(getErrorInfo(504), {
+    type: "server_error",
+    code: "gateway_timeout",
+  });
+});
+
+test("errorConfig resolves text rules before status rules", () => {
+  const textRule = matchErrorRuleByText("Rate limit reached");
+  assert.equal(textRule?.id, "rate_limit");
+  assert.equal(textRule?.backoff, true);
+
+  const statusRule = matchErrorRuleByStatus(404);
+  assert.equal(statusRule?.id, "status_404");
+  assert.equal(statusRule?.cooldownMs, COOLDOWN_MS.notFound);
+
+  const combinedRule = findMatchingErrorRule(429, "Request not allowed by upstream");
+  assert.equal(combinedRule?.id, "request_not_allowed");
+  assert.equal(combinedRule?.cooldownMs, COOLDOWN_MS.requestNotAllowed);
+});
+
+test("errorConfig preserves the existing exponential backoff policy", () => {
+  assert.equal(calculateBackoffCooldown(0), BACKOFF_CONFIG.base);
+  assert.equal(calculateBackoffCooldown(3), BACKOFF_CONFIG.base * 8);
+  assert.equal(calculateBackoffCooldown(BACKOFF_CONFIG.maxLevel + 20), BACKOFF_CONFIG.max);
+});