fix(agent): 视频生成 - 支持 Agent 设置首尾帧 (#696)

* fix(agent): 视频生成 - 支持 Agent 设置首尾帧

* fix(agent): 工具参数 - 精简首尾帧说明并保持体积预算

---------

Co-authored-by: tanzui <305375675@.com>

backend/internal/app/cloud_agent_generation_contract.go

@@ -5,8 +5,8 @@ import (
 	"strings"
 )
 
-// Agent arguments and the canvas editor enter the same public generation codec.
-// Resource materialization and provider credentials remain outside this object.
+// cloudAgentGenerationSpec 将 Agent 参数和已解析素材转换为画布生成合同。
+// refs 提供真实素材类型，resolvedConfig 提供准入后的模型默认值；返回可保存的规格或参数错误。
 func cloudAgentGenerationSpec(a cloudAgentMediaArgs, refs map[string]any, resolvedConfig map[string]any) (contract.GenerationSpec, error) {
 	selection := &contract.ModelSelection{Kind: "channel", ChannelID: a.ChannelID, ModelKey: a.ChannelModelKey}
 	if a.LogicalModelID != "" {
@@ -59,6 +59,19 @@ func cloudAgentGenerationSpec(a cloudAgentMediaArgs, refs map[string]any, resolv
 		}
 	}
 	bindings := []contract.ReferenceBinding{}
+	// 帧选择沿用视频编辑器的 metadata 字段；这里只校验真实引用，不从提示词或图片顺序猜用途。
+	for _, frame := range []struct{ field, id string }{
+		{"videoStartFrameNodeId", a.VideoStartFrameNodeID},
+		{"videoEndFrameNodeId", a.VideoEndFrameNodeID},
+	} {
+		if frame.id == "" {
+			continue
+		}
+		binding, exists := byID[frame.id]
+		if a.Mode != "video" || !exists || binding.MediaType != "image" || !containsString(a.ReferenceNodeIDs, frame.id) {
+			return contract.GenerationSpec{}, cloudAgentFieldError(frame.field, "invalid_value", "首尾帧仅用于视频，必须选择 referenceNodeIds 中的真实图片节点")
+		}
+	}
 	for _, id := range a.ReferenceNodeIDs {
 		binding, exists := byID[id]
 		if !exists {