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/httpapi/client_error_detail_test.go

@@ -0,0 +1,26 @@
+package httpapi
+
+import (
+	"testing"
+
+	"github.com/easyai/easyai-ai-gateway/apps/api/internal/clients"
+)
+
+func TestRunErrorDetailsExposeStructuredClientFailure(t *testing.T) {
+	err := &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",
+		},
+	}
+
+	details := runErrorDetails(err)
+	if details["provider"] != "gemini" ||
+		details["reason"] != "prompt_blocked" ||
+		details["promptBlockReason"] != "SAFETY" {
+		t.Fatalf("public error details lost Gemini extraction diagnostics: %+v", details)
+	}
+}