fix(gateway): restore safe search fallback

Fall back to guarded Chat Completions answers when DeepSeek web search fails so grounded current-information hints do not surface generic provider errors.

src/main/kotlin/com/osglab/account/features/gateway/providers/deepseek/DeepSeekResponsesClient.kt

@@ -9,6 +9,7 @@ import com.osglab.account.features.gateway.models.ProviderOutput
 import com.osglab.account.features.gateway.models.ProviderUsage
 import com.osglab.account.features.gateway.models.TextProviderRequest
 import com.osglab.account.features.gateway.models.UsageMeter
+import com.osglab.account.features.gateway.providers.ProviderUpstreamException
 import io.ktor.client.HttpClient
 import io.ktor.client.call.body
 import io.ktor.client.request.bearerAuth
@@ -72,16 +73,16 @@ internal class DeepSeekSearchFallbackClient(
         } catch (failure: CancellationException) {
             throw failure
         } catch (failure: Exception) {
+            val upstreamStatus = (failure as? ProviderUpstreamException)?.upstreamStatus
             LOG.warn(
-                "DeepSeek search path failed requestId={} taskKind={} searchMode={} failureType={}",
+                "DeepSeek search path failed requestId={} taskKind={} searchMode={} " +
+                    "failureType={} upstreamStatus={} fallback=chat_completions",
                 request.requestId,
                 request.executionPolicy.taskKind.name,
                 request.executionPolicy.webSearch.name,
                 failure::class.simpleName ?: "Exception",
+                upstreamStatus ?: "unknown",
             )
-            if (request.executionPolicy.webSearch == GatewayWebSearchMode.REQUIRED) {
-                throw failure
-            }
             return fallback.complete(
                 request.copy(
                     executionPolicy = request.executionPolicy.copy(

src/test/kotlin/com/osglab/account/features/gateway/providers/deepseek/DeepSeekClientTest.kt

@@ -291,16 +291,28 @@ class DeepSeekClientTest : StringSpec({
         }
     }
 
-    "does not return an offline answer when current information requires search" {
+    "falls back to a guarded answer when required search fails" {
         val paths = mutableListOf<String>()
+        val requestBodies = mutableListOf<String>()
         val client = HttpClient(
             MockEngine { request ->
                 paths += request.url.encodedPath
-                respond(
-                    content = """{"error":{"message":"search unavailable"}}""",
-                    status = HttpStatusCode.ServiceUnavailable,
-                    headers = headersOf(HttpHeaders.ContentType, ContentType.Application.Json.toString()),
-                )
+                requestBodies += request.body.toByteArray().decodeToString()
+                if (request.url.encodedPath.endsWith("/responses")) {
+                    respond(
+                        content = """{"error":{"message":"search unavailable"}}""",
+                        status = HttpStatusCode.ServiceUnavailable,
+                        headers = headersOf(HttpHeaders.ContentType, ContentType.Application.Json.toString()),
+                    )
+                } else {
+                    respond(
+                        content =
+                            """{"choices":[{"message":{"content":"无法核实实时信息"}}],""" +
+                                """"usage":{"prompt_tokens":9,"completion_tokens":4,"total_tokens":13}}""",
+                        status = HttpStatusCode.OK,
+                        headers = headersOf(HttpHeaders.ContentType, ContentType.Application.Json.toString()),
+                    )
+                }
             },
         ) {
             install(ContentNegotiation) {
@@ -308,17 +320,20 @@ class DeepSeekClientTest : StringSpec({
             }
         }
         try {
-            shouldThrow<DeepSeekProviderException> {
-                DeepSeekProvider(client, CONFIG).execute(
-                    request(
-                        taskKind = GatewayTaskKind.CURRENT_INFORMATION_QUESTION,
-                        webSearch = GatewayWebSearchMode.REQUIRED,
-                    ),
-                    DISCARD_OUTPUT,
-                )
-            }
+            val usage = DeepSeekProvider(client, CONFIG).execute(
+                request(
+                    taskKind = GatewayTaskKind.CURRENT_INFORMATION_QUESTION,
+                    webSearch = GatewayWebSearchMode.REQUIRED,
+                ),
+                DISCARD_OUTPUT,
+            )
 
-            paths shouldBe listOf("/v1/responses")
+            paths shouldBe listOf("/v1/responses", "/v1/chat/completions")
+            val fallbackSystem = Json.parseToJsonElement(requestBodies.last()).jsonObject
+                .getValue("messages").jsonArray.first().jsonObject
+                .getValue("content").jsonPrimitive.content
+            fallbackSystem shouldContain "could not be verified"
+            usage.units shouldBe 13
         } finally {
             client.close()
         }