fix: avoid mutating frozen Hit objects in cacheOnFail path

Author: andrewshell

src/common.ts

@@ -33,6 +33,16 @@ export class Hit {
     this.data = data;
     this.etag = existingEtag ?? generateEtag(JSON.stringify(data));
   }
+
+  withError(error: string, consecutiveErrors: number): Hit {
+    const hit = new Hit(this.cacheName, this.data, this.etag);
+    hit.cached = this.cached;
+    hit.created = this.created;
+    hit.error = error;
+    hit.errorTime = new Date();
+    hit.consecutiveErrors = consecutiveErrors;
+    return hit;
+  }
 }
 
 export class Miss {

src/index.ts

@@ -63,10 +63,7 @@ export class Cacheism {
       const errorMessage = err instanceof Error ? err.toString() : String(err);
 
       if (status >= Status.cacheOnFail && hasCache && existing.isHit) {
-        response = existing;
-        response.error = errorMessage;
-        response.errorTime = new Date();
-        response.consecutiveErrors++;
+        response = existing.withError(errorMessage, existing.consecutiveErrors + 1);
       } else {
         response = new Miss(name, errorMessage, existing.consecutiveErrors + 1);
       }