fix(web): close P3 chat re-review findings

apps/web/src/spa/chat/runtime-guards.ts

@@ -10,6 +10,14 @@ export function asString(value: unknown, fallback = ''): string {
   return typeof value === 'string' ? value : fallback;
 }
 
+/** Like `asString`, but an empty string also falls back — used for guarded
+ * contract-provided reason strings (e.g. a denial or failure message) where
+ * an empty string is not a meaningful value to display in place of the
+ * stable fallback copy. */
+export function asNonEmptyString(value: unknown, fallback: string): string {
+  return typeof value === 'string' && value.length > 0 ? value : fallback;
+}
+
 export function asFiniteNumber(value: unknown, fallback = 0): number {
   return typeof value === 'number' && Number.isFinite(value) ? value : fallback;
 }