Fix Spotlight indexing: route the entity URL to contentURL

Every bookmark failed to reach the Spotlight index. Each item died in
translation with "Provided object for field url is of class NSURL,
expected class: NSString", so on-device search and Ask Your Bookmarks
were retrieving from an index that was effectively empty.

Left to itself, App Intents indexes BookmarkEntity's `url` property under
the attribute set's own `url` key, which Spotlight's Cascade translator
types as NSString. Giving the property an explicit
`indexingKey: \.contentURL` sends it to a URL-typed field instead — and
contentURL is the right field for "where this content lives" regardless.
Keeping the property a URL rather than retyping it to String means
existing Shortcuts that read it are unaffected.

The attribute set now sets contentURL too, and the retrieval side reads
it back, so the round trip stays on one field.

Why it went unnoticed: translation happens after `indexAppEntities`
returns, so indexing logged success the whole time. Measured on the
simulator against the live library — 202 translation failures per launch
before, 0 after.

Adds SpotlightRetrievalTests, which indexes a bookmark and retrieves it
through the assistant's own path. Nothing weaker would have caught this,
since the failure was silent at every layer above the index.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JgLHztZGaEHvS3KNeGQmRM

Marks/Services/SpotlightBookmarkSearch.swift

@@ -24,7 +24,7 @@ enum SpotlightBookmarkSearch {
         Log.spotlight.debug("Search query=\(rawQuery, privacy: .public) predicate=\(queryString, privacy: .public)")
 
         let context = CSSearchQueryContext()
-        context.fetchAttributes = ["title", "contentDescription", "keywords", "url"]
+        context.fetchAttributes = ["title", "contentDescription", "keywords", "contentURL"]
         let query = CSSearchQuery(queryString: queryString, queryContext: context)
 
         var out: [RetrievedBookmark] = []
@@ -33,9 +33,9 @@ enum SpotlightBookmarkSearch {
                 let a = result.item.attributeSet
                 out.append(RetrievedBookmark(
                     title: a.title ?? "Untitled",
-                    host: a.url?.host() ?? "",
+                    host: a.contentURL?.host() ?? "",
                     description: a.contentDescription ?? "",
-                    url: a.url?.absoluteString ?? ""
+                    url: a.contentURL?.absoluteString ?? ""
                 ))
                 if out.count >= limit { break }
             }