fix(gemini): 移除空返图占位并暴露失败原因

Gemini 图片响应未包含可提取资源时，不再返回伪造占位图，改为非重试失败并保留安全拦截、候选结束状态和上游错误等结构化诊断。\n\n同步将诊断写入 HTTP 错误、任务结果、attempt 指标和日志，确保任务失败时不结算，并补充单元及 PostgreSQL 验收测试。\n\n验证：gofmt、go test ./... -count=1、go vet ./...、迁移安全检查、pnpm openapi。

apps/api/internal/runner/service.go

@@ -1412,6 +1412,20 @@ func (s *Service) runCandidate(
 			response.ResponseDurationMS = 0
 		}
 	}
+	if err != nil && clients.ErrorCode(err) == "gemini_image_output_missing" && s.logger != nil {
+		meta := clients.ErrorResponseMetadata(err)
+		s.logger.Warn("gemini image response contains no extractable resource",
+			"taskID", task.ID,
+			"attempt", attemptNo,
+			"platformID", candidate.PlatformID,
+			"platformModelID", candidate.PlatformModelID,
+			"clientID", candidate.ClientID,
+			"requestID", meta.RequestID,
+			"error_category", clients.ErrorCode(err),
+			"reason", err.Error(),
+			"details", clients.ErrorDetails(err),
+		)
+	}
 	if err != nil {
 		if clients.ErrorResponseMetadata(err).StatusCode > 0 && submissionStatus != "response_received" {
 			if markErr := setSubmissionStatus("response_received"); markErr != nil {