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/recording_test.go

@@ -3,6 +3,7 @@ package runner
 import (
 	"testing"
 
+	"github.com/easyai/easyai-ai-gateway/apps/api/internal/clients"
 	"github.com/easyai/easyai-ai-gateway/apps/api/internal/store"
 )
 
@@ -45,3 +46,27 @@ func TestAttemptMetricsIncludesCacheAffinityCoefficient(t *testing.T) {
 		t.Fatalf("expected cache affinity routing diagnostics in attempt metrics, got %+v", metrics)
 	}
 }
+
+func TestFailureRecordIncludesStructuredClientErrorDetails(t *testing.T) {
+	cause := &clients.ClientError{
+		Code:    "gemini_image_output_missing",
+		Message: "Gemini image response contains no extractable image: prompt blocked",
+		Details: map[string]any{
+			"provider":          "gemini",
+			"reason":            "prompt_blocked",
+			"promptBlockReason": "SAFETY",
+		},
+	}
+
+	_, metrics, _, _, _ := failureMetrics(cause, false)
+	details, _ := metrics["errorDetails"].(map[string]any)
+	if details["reason"] != "prompt_blocked" || details["promptBlockReason"] != "SAFETY" {
+		t.Fatalf("failure metrics lost client error details: %+v", metrics)
+	}
+	result := buildFailureResult(cause.Code, cause.Message, "", cause)
+	errorPayload, _ := result["error"].(map[string]any)
+	resultDetails, _ := errorPayload["details"].(map[string]any)
+	if resultDetails["provider"] != "gemini" || resultDetails["reason"] != "prompt_blocked" {
+		t.Fatalf("failure result lost client error details: %+v", result)
+	}
+}