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/embeddings.ts

@@ -36,10 +36,11 @@ export type EmbeddingProviderFallback = EmbeddingProviderId | "none";
 const REMOTE_EMBEDDING_PROVIDER_IDS = ["openai", "gemini", "voyage"] as const;
 
 export type EmbeddingProviderResult = {
-  provider: EmbeddingProvider;
+  provider: EmbeddingProvider | null;
   requestedProvider: EmbeddingProviderRequest;
   fallbackFrom?: EmbeddingProviderId;
   fallbackReason?: string;
+  providerUnavailableReason?: string;
   openAi?: OpenAiEmbeddingClient;
   gemini?: GeminiEmbeddingClient;
   voyage?: VoyageEmbeddingClient;
@@ -183,15 +184,19 @@ export async function createEmbeddingProvider(
           missingKeyErrors.push(message);
           continue;
         }
+        // Non-auth errors (e.g., network) are still fatal
         throw new Error(message, { cause: err });
       }
     }
 
+    // All providers failed due to missing API keys - return null provider for FTS-only mode
     const details = [...missingKeyErrors, localError].filter(Boolean) as string[];
-    if (details.length > 0) {
-      throw new Error(details.join("\n\n"));
-    }
-    throw new Error("No embeddings provider available.");
+    const reason = details.length > 0 ? details.join("\n\n") : "No embeddings provider available.";
+    return {
+      provider: null,
+      requestedProvider,
+      providerUnavailableReason: reason,
+    };
   }
 
   try {
@@ -209,13 +214,31 @@ export async function createEmbeddingProvider(
           fallbackReason: reason,
         };
       } catch (fallbackErr) {
-        // oxlint-disable-next-line preserve-caught-error
-        throw new Error(
-          `${reason}\n\nFallback to ${fallback} failed: ${formatErrorMessage(fallbackErr)}`,
-          { cause: fallbackErr },
-        );
+        // Both primary and fallback failed - check if it's auth-related
+        const fallbackReason = formatErrorMessage(fallbackErr);
+        const combinedReason = `${reason}\n\nFallback to ${fallback} failed: ${fallbackReason}`;
+        if (isMissingApiKeyError(primaryErr) && isMissingApiKeyError(fallbackErr)) {
+          // Both failed due to missing API keys - return null for FTS-only mode
+          return {
+            provider: null,
+            requestedProvider,
+            fallbackFrom: requestedProvider,
+            fallbackReason: reason,
+            providerUnavailableReason: combinedReason,
+          };
+        }
+        // Non-auth errors are still fatal
+        throw new Error(combinedReason, { cause: fallbackErr });
       }
     }
+    // No fallback configured - check if we should degrade to FTS-only
+    if (isMissingApiKeyError(primaryErr)) {
+      return {
+        provider: null,
+        requestedProvider,
+        providerUnavailableReason: reason,
+      };
+    }
     throw new Error(reason, { cause: primaryErr });
   }
 }