[w3c/ServiceWorker] Static routing: the "cache" source stops at the first cache that lacks the request (Issue #1853)

bcardarella created an issue (w3c/ServiceWorker#1853)

In Handle Fetch (https://w3c.github.io/ServiceWorker/#handle-fetch), step 13.3.3.7 walks the storage key's name to cache map for a "cache" router source:

> 2. Let requestResponses be the result of running Query Cache with request, a new CacheQueryOptions, and cache.
> 3. If requestResponses is an empty list, return timingInfo.

Returning there sends the request to the network at the first cache with no match, even when a later cache has a response. With `source: {cacheName}` only one cache is queried (step 7.1), so this affects `source: "cache"` only.

Implementations search every cache, as `caches.match()` does:
- Chromium: ServiceWorkerCacheStorageMatcher::Run calls Cache Storage's Match with MultiCacheQueryOptions and no cache name, which is CacheStorage::MatchAllCaches: every cache in order, the first match wins.
  https://source.chromium.org/chromium/chromium/src/+/main:content/browser/service_worker/service_worker_cache_storage_matcher.cc
  https://source.chromium.org/chromium/chromium/src/+/main:content/browser/cache_storage/cache_storage.cc (MatchAllCachesImpl, MatchAllCachesDidMatchAll)
- WebKit (Safari 27): CacheStorageManager::query with an empty name calls queryCaches, which moves to the next cache when one has no match.
  https://github.com/WebKit/WebKit/blob/main/Source/WebKit/NetworkProcess/storage/CacheStorageManager.cpp
- Gecko does not ship addRoutes (https://bugzil.la/1855580).

#1797 also describes "cache" as matching against all caches.

Scenario, in the install handler:

```js
await (await caches.open('a')).put('/other', new Response('other'));
await (await caches.open('b')).put('/data', new Response('from cache'));
event.addRoutes({condition: {urlPattern: '/data'}, source: 'cache'});
```

A controlled page's `fetch('/data')`: by the spec text, cache "a" has no match, so the request goes to the network. Chrome and Safari answer "from cache" from cache "b".

This shows up through leftover state: WPT's static-router tests use one cache, "v1", and fail in an engine that follows the text when an earlier test in the same browser session left another cache behind (resource-timing-cross-origin-server-timing.https.html leaves "test-cache").

Suggested change: step 13.3.3.7.3 becomes "If requestResponses is an empty list, continue." The existing step 13.3.3.8 ("Return timingInfo") then covers the case where no cache matches.


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