Netlify preset: defineCachedEventHandler doesn't emit Netlify-Vary: query, causing query-variant cache collapse

Environment

nitro v2, using netlify-edge preset.
The issue should also happen with netlify functions.

Reproduction

I spotted the bug using @nuxt/icon but the issue is broader.

@nuxt/icon registers /api/_nuxt_icon/[collection].json using:

defineCachedEventHandler(handler, {
  getKey(event) {                                                                                                                                                                    
    const icons = String(getQuery(event).icons || "");
    return `${collection}_${icons.split(",")[0]}_${icons.length}_${hash(icons)}`;                                                                                                    
  },                                                                                                                                                                                 
  swr: true,                                                                                                                                                                         
  maxAge: 60 * 60 * 24 * 7,                                                                                                                                                          
});             

The handler returns different JSON depending on the ?icons= query param. Nitro emits:

  cache-control: s-maxage=604800, stale-while-revalidate                                                                                                                               

Netlify CDN caches this. The next request to the same path with a different ?icons= value gets the cached response from the first request — wrong icons.

Describe the bug

When using the Netlify preset, defineCachedEventHandler with a query-based getKey emits cache-control: s-maxage=..., stale-while-revalidate — which Netlify CDN honours by caching the response. But because no Netlify-Vary: query header is present, the CDN collapses all query string variants into a single cache entry, serving the first-cached response for every subsequent variant.

Nitro's server-side cache works correctly (it keys by the query-derived value). The CDN layer silently breaks it.

I managed to work around the issue by adding the netlify-vary header manually in Nuxt's routeRules:

  // nuxt.config.ts                                                                                                                                                                    
  routeRules: {   
    '/api/_nuxt_icon/*': {
      headers: { 'Netlify-Vary': 'query' },
    },
  }   

Additional context

No response

Logs

Start by tracing the Netlify preset handling for defineCachedEventHandler and how response headers are generated. Reproduce the issue with two query-string variants, then verify that the Netlify-Vary: query header is emitted and that a regression test covers distinct cached responses.