fix(prompt-gate): harden internal prompt dispatch

src/shared/prompt-failure-classifier.ts

@@ -0,0 +1,24 @@
+export function extractPromptFailureMessage(error: unknown): string {
+  if (typeof error === "string") return error
+  if (error instanceof Error) return error.message
+  if (typeof error === "object" && error !== null) {
+    const record = error as Record<string, unknown>
+    if (typeof record.message === "string") return record.message
+    try {
+      return JSON.stringify(error)
+    } catch {
+      return ""
+    }
+  }
+  return String(error)
+}
+
+export function isAmbiguousPromptDispatchFailure(error: unknown): boolean {
+  const message = extractPromptFailureMessage(error).toLowerCase()
+  return (
+    message.includes("unexpected eof")
+    || message.includes("json parse error")
+    || message.includes("unexpected end of json input")
+    || message.includes("timed out")
+  )
+}