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/agents/tools/memory-tool.ts

@@ -81,12 +81,14 @@ export function createMemorySearchTool(options: {
           status.backend === "qmd"
             ? clampResultsByInjectedChars(decorated, resolved.qmd?.limits.maxInjectedChars)
             : decorated;
+        const searchMode = (status.custom as { searchMode?: string } | undefined)?.searchMode;
         return jsonResult({
           results,
           provider: status.provider,
           model: status.model,
           fallback: status.fallback,
           citations: citationsMode,
+          mode: searchMode,
         });
       } catch (err) {
         const message = err instanceof Error ? err.message : String(err);