perf(acceptance): 流式生成唯一图片输入

避免千请求验收重复复用同一图片内容而争用同一资产记录，使每个任务走真实独立上传与物化链路。\n\nGemini 请求体改为流式 Base64 编码，降低压测客户端的大对象内存峰值；新增尺寸、哈希唯一性和请求体还原测试。\n\n验证：Go 全量测试、go vet、迁移安全检查、bash -n、ShellCheck、gofmt、git diff --check 均通过。

apps/api/cmd/acceptance-load/main_test.go

@@ -27,6 +27,58 @@ func TestStreamGeminiImageHashDoesNotNeedWholeResponse(t *testing.T) {
 	}
 }
 
+func TestPaddedPNGVariantsAreExactSizeAndUnique(t *testing.T) {
+	first := paddedPNGVariant(256<<10, 1)
+	second := paddedPNGVariant(256<<10, 2)
+	if len(first) != 256<<10 || len(second) != 256<<10 {
+		t.Fatalf("variant sizes=%d/%d", len(first), len(second))
+	}
+	firstHash := sha256.Sum256(first)
+	secondHash := sha256.Sum256(second)
+	if firstHash == secondHash {
+		t.Fatal("distinct variants have the same SHA-256")
+	}
+}
+
+func TestStreamGeminiRequestBodyPreservesInput(t *testing.T) {
+	input := paddedPNGVariant(2<<20, 37)
+	var body struct {
+		Contents []struct {
+			Parts []struct {
+				InlineData *struct {
+					MIMEType string `json:"mimeType"`
+					Data     string `json:"data"`
+				} `json:"inlineData"`
+			} `json:"parts"`
+		} `json:"contents"`
+		GenerationConfig struct {
+			ResponseModalities []string `json:"responseModalities"`
+		} `json:"generationConfig"`
+	}
+	stream := streamGeminiRequestBody(input)
+	defer stream.Close()
+	if err := json.NewDecoder(stream).Decode(&body); err != nil {
+		t.Fatalf("decode streamed Gemini request: %v", err)
+	}
+	if len(body.Contents) != 1 || len(body.Contents[0].Parts) != 2 || body.Contents[0].Parts[1].InlineData == nil {
+		t.Fatalf("unexpected Gemini body structure: %+v", body)
+	}
+	inlineData := body.Contents[0].Parts[1].InlineData
+	if inlineData.MIMEType != "image/png" {
+		t.Fatalf("mime type=%q", inlineData.MIMEType)
+	}
+	decoded, err := base64.StdEncoding.DecodeString(inlineData.Data)
+	if err != nil {
+		t.Fatalf("decode streamed input: %v", err)
+	}
+	if !bytes.Equal(decoded, input) {
+		t.Fatal("streamed input differs from source")
+	}
+	if len(body.GenerationConfig.ResponseModalities) != 1 || body.GenerationConfig.ResponseModalities[0] != "IMAGE" {
+		t.Fatalf("response modalities=%v", body.GenerationConfig.ResponseModalities)
+	}
+}
+
 func TestVideoCombinationsProvide128UniqueInputs(t *testing.T) {
 	images := make([]string, 16)
 	for index := range images {