fix(deps): update dependency @sanity/tsdown-config to ^0.28.7

plugins/@sanity/debug-preview-url-secret-plugin/tsdown.config.ts

@@ -1,4 +1,8 @@
 import {defineConfig} from '@sanity/tsdown-config'
 import type {UserConfig} from 'tsdown'
 
-export default defineConfig() satisfies Promise<UserConfig | UserConfig[]>
+// The plugin's tsdown and the copy nested under @sanity/tsdown-config are different
+// peer instances, so an inferred export type names UserConfig from a non-portable path.
+const config: Promise<UserConfig | UserConfig[]> = defineConfig()
+
+export default config