fix(agents): trigger model failover on connection-refused and network-unreachable errors

Previously, only ETIMEDOUT / ESOCKETTIMEDOUT / ECONNRESET / ECONNABORTED
were recognised as failover-worthy network errors. Connection-level
failures such as ECONNREFUSED (server down), ENETUNREACH / EHOSTUNREACH
(network disconnected), ENETRESET, and EAI_AGAIN (DNS failure) were
treated as unknown errors and did not advance the fallback chain.

This is particularly impactful when a local fallback model (e.g. Ollama)
is configured: if the remote provider is unreachable due to a network
outage, the gateway should fall back to the local model instead of
returning an error to the user.

Add the missing error codes to resolveFailoverReasonFromError() and
corresponding e2e tests.

Closes #18868

src/agents/failover-error.ts

@@ -183,7 +183,19 @@ export function resolveFailoverReasonFromError(err: unknown): FailoverReason | n
   }
 
   const code = (getErrorCode(err) ?? "").toUpperCase();
-  if (["ETIMEDOUT", "ESOCKETTIMEDOUT", "ECONNRESET", "ECONNABORTED"].includes(code)) {
+  if (
+    [
+      "ETIMEDOUT",
+      "ESOCKETTIMEDOUT",
+      "ECONNRESET",
+      "ECONNABORTED",
+      "ECONNREFUSED",
+      "ENETUNREACH",
+      "EHOSTUNREACH",
+      "ENETRESET",
+      "EAI_AGAIN",
+    ].includes(code)
+  ) {
     return "timeout";
   }
   if (isTimeoutError(err)) {