feat(FN-2718): add unavailable node policy setting and validation

- Define and export unavailable node policy types in core settings interfaces
- Add project-level unavailable node policy default and runtime validation helper
- Add core unit coverage for unavailable node policy parsing and acceptance cases
- Guard Paperclip mint requests to include companyId only when available for type-safe payloads
- Document unavailable node policy in the settings reference

packages/core/src/settings-validation.ts

@@ -0,0 +1,20 @@
+import type { UnavailableNodePolicy } from "./types.js";
+
+const UNAVAILABLE_NODE_POLICIES: readonly UnavailableNodePolicy[] = ["block", "fallback-local"] as const;
+
+/**
+ * Validates a project unavailable-node routing policy value.
+ *
+ * Returns the normalized policy value when valid, otherwise undefined.
+ */
+export function validateUnavailableNodePolicy(value: unknown): UnavailableNodePolicy | undefined {
+  if (value === undefined) {
+    return undefined;
+  }
+  if (typeof value !== "string") {
+    return undefined;
+  }
+  return (UNAVAILABLE_NODE_POLICIES as readonly string[]).includes(value)
+    ? (value as UnavailableNodePolicy)
+    : undefined;
+}