harden(ai): ModelStore seedFromBundle 在 DEBUG 报错,加空目录测试

按 code quality review 反馈:
- seedFromBundle 找不到 bundle 资源时,DEBUG 下 assertionFailure 提示
  target membership(release 仍静默 return),避免 W6 启用时排查困难
- 补 totalBytesReturnsZeroWhenFolderMissing 测试,覆盖 folder 不存在时
  enumerator 为 nil 的 guard 路径

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

体己Tests/ModelStoreTests.swift

@@ -43,4 +43,12 @@ struct ModelStoreTests {
 
         #expect(store.totalBytes(for: .llm) == 2048)
     }
+
+    @Test func totalBytesReturnsZeroWhenFolderMissing() throws {
+        let store = try isolatedStore()
+        defer { try? FileManager.default.removeItem(at: store.rootURL) }
+
+        // 没创建任何 vl 目录
+        #expect(store.totalBytes(for: .vl) == 0)
+    }
 }