fix(studio): name the file store search engine scan, not memory

The provider guard test refuses core code that names an add-on, and
"memory" is the name of one. The file store searched its in-memory
transcripts and reported the engine as "memory". It now reports "scan",
as every store without an index does.

Assisted-by: Claude:claude-sonnet-5-5

core/schema/chats_features.go

@@ -71,7 +71,7 @@ type ChatsChatHit struct {
 // ChatsChatSearchResponse is the response of the chat search.
 type ChatsChatSearchResponse struct {
 	Hits []ChatsChatHit `json:"hits"`
-	// Engine is how the server searched: fts5, pg_trgm, scan or memory.
+	// Engine is how the server searched: fts5, pg_trgm or scan.
 	Engine string `json:"engine,omitempty"`
 	// Indexing is true while the server still adds earlier chats to its
 	// search index, so older chats may be missing from the hits.

core/services/studio/chatsearch/chatsearch.go

@@ -120,7 +120,7 @@ type Progress struct {
 // Result is the answer of a search.
 type Result struct {
 	Hits []Hit
-	// Engine is how the store searched: fts5, pg_trgm, scan or memory.
+	// Engine is how the store searched: fts5, pg_trgm or scan.
 	Engine string
 	// Partial is true when a time budget ended the search before every chat
 	// was read (a store without an index).

core/services/studio/store/jsonsearch.go

@@ -13,7 +13,7 @@ import (
 // scans them, the newest chat first, until chatsearch.ScanBudget is spent.
 // Result.Searched and Total say how many chats it read.
 func (s *jsonStore) SearchText(ctx context.Context, q chatsearch.Query) (chatsearch.Result, error) {
-	res := chatsearch.Result{Engine: "memory"}
+	res := chatsearch.Result{Engine: engineScan}
 	terms := q.Terms()
 	if len(terms) == 0 {
 		return res, nil