fix: enable FTS fallback when no embedding provider available (#17725)

When no embedding provider is available (e.g., OAuth mode without API keys),
memory_search now falls back to FTS-only mode instead of returning disabled: true.

Changes:
- embeddings.ts: return null provider with reason instead of throwing
- manager.ts: handle null provider, use FTS-only search mode
- manager-search.ts: allow searching all models when provider is undefined
- memory-tool.ts: expose search mode in results

The search results now include a 'mode' field indicating 'hybrid' or 'fts-only'.

src/memory/manager-search.ts

@@ -136,7 +136,7 @@ export function listChunks(params: {
 export async function searchKeyword(params: {
   db: DatabaseSync;
   ftsTable: string;
-  providerModel: string;
+  providerModel: string | undefined;
   query: string;
   limit: number;
   snippetMaxChars: number;
@@ -152,16 +152,20 @@ export async function searchKeyword(params: {
     return [];
   }
 
+  // When providerModel is undefined (FTS-only mode), search all models
+  const modelClause = params.providerModel ? " AND model = ?" : "";
+  const modelParams = params.providerModel ? [params.providerModel] : [];
+
   const rows = params.db
     .prepare(
       `SELECT id, path, source, start_line, end_line, text,\n` +
         `       bm25(${params.ftsTable}) AS rank\n` +
         `  FROM ${params.ftsTable}\n` +
-        ` WHERE ${params.ftsTable} MATCH ? AND model = ?${params.sourceFilter.sql}\n` +
+        ` WHERE ${params.ftsTable} MATCH ?${modelClause}${params.sourceFilter.sql}\n` +
         ` ORDER BY rank ASC\n` +
         ` LIMIT ?`,
     )
-    .all(ftsQuery, params.providerModel, ...params.sourceFilter.params, params.limit) as Array<{
+    .all(ftsQuery, ...modelParams, ...params.sourceFilter.params, params.limit) as Array<{
     id: string;
     path: string;
     source: SearchSource;