Implement checkpoint blob handling in forwarder service

- Added support for checkpointing phases and blob management in the forwarder.
- Introduced new types and methods for handling checkpoint blobs, including queuing and publishing checkpoints.
- Enhanced the projector to build checkpoint projections with content-addressed blobs.
- Implemented tests to ensure proper checkpoint blob synchronization and handling of cancellation scenarios.

internal/backend/forwarder/events.go

@@ -245,6 +245,22 @@ func buildCheckpointMessage(state *agentv1.ConversationStateStructure) *agentv1.
 	}
 }
 
+func buildSetCheckpointBlobMessage(id uint32, blob CheckpointBlob) *agentv1.AgentServerMessage {
+	return &agentv1.AgentServerMessage{
+		Message: &agentv1.AgentServerMessage_KvServerMessage{
+			KvServerMessage: &agentv1.KvServerMessage{
+				Id: id,
+				Message: &agentv1.KvServerMessage_SetBlobArgs{
+					SetBlobArgs: &agentv1.SetBlobArgs{
+						BlobId:   append([]byte(nil), blob.ID...),
+						BlobData: append([]byte(nil), blob.Data...),
+					},
+				},
+			},
+		},
+	}
+}
+
 // buildExecAbortMessage 构造对客户端执行桥的 abort 控制消息。
 func buildExecAbortMessage(pending runtimecore.PendingExec) *agentv1.AgentServerMessage {
 	return &agentv1.AgentServerMessage{