refactor: 实现 ConversionEngine 协议转换引擎，替代旧 protocol 包

引入 Canonical Model 和 ProtocolAdapter 架构，支持 OpenAI/Anthropic 协议间
无缝转换，统一 ProxyHandler 替代分散的 OpenAI/Anthropic Handler，简化
ProviderClient 为协议无关的 HTTP 发送器，Provider 新增 protocol 字段。

backend/internal/conversion/canonical/stream.go

@@ -0,0 +1,156 @@
+package canonical
+
+// StreamEventType 流式事件类型枚举
+type StreamEventType string
+
+const (
+	EventMessageStart      StreamEventType = "message_start"
+	EventContentBlockStart StreamEventType = "content_block_start"
+	EventContentBlockDelta StreamEventType = "content_block_delta"
+	EventContentBlockStop  StreamEventType = "content_block_stop"
+	EventMessageDelta      StreamEventType = "message_delta"
+	EventMessageStop       StreamEventType = "message_stop"
+	EventError             StreamEventType = "error"
+	EventPing              StreamEventType = "ping"
+)
+
+// DeltaType 增量类型枚举
+type DeltaType string
+
+const (
+	DeltaTypeText        DeltaType = "text_delta"
+	DeltaTypeInputJSON   DeltaType = "input_json_delta"
+	DeltaTypeThinking    DeltaType = "thinking_delta"
+)
+
+// StreamDelta 流式增量联合体
+type StreamDelta struct {
+	Type         string `json:"type"`
+	Text         string `json:"text,omitempty"`
+	PartialJSON  string `json:"partial_json,omitempty"`
+	Thinking     string `json:"thinking,omitempty"`
+}
+
+// StreamContentBlock 流式内容块联合体
+type StreamContentBlock struct {
+	Type     string `json:"type"`
+	Text     string `json:"text,omitempty"`
+	ID       string `json:"id,omitempty"`
+	Name     string `json:"name,omitempty"`
+	Thinking string `json:"thinking,omitempty"`
+}
+
+// CanonicalStreamEvent 规范流式事件联合体
+type CanonicalStreamEvent struct {
+	Type StreamEventType `json:"type"`
+
+	// MessageStartEvent
+	Message *StreamMessage `json:"message,omitempty"`
+
+	// ContentBlockStartEvent / ContentBlockDeltaEvent / ContentBlockStopEvent
+	Index        *int              `json:"index,omitempty"`
+	ContentBlock *StreamContentBlock `json:"content_block,omitempty"`
+	Delta        *StreamDelta      `json:"delta,omitempty"`
+
+	// MessageDeltaEvent
+	StopReason *StopReason    `json:"stop_reason,omitempty"`
+	Usage      *CanonicalUsage `json:"usage,omitempty"`
+
+	// ErrorEvent
+	Error *StreamError `json:"error,omitempty"`
+}
+
+// StreamMessage 流式消息摘要
+type StreamMessage struct {
+	ID    string          `json:"id"`
+	Model string          `json:"model"`
+	Usage *CanonicalUsage `json:"usage,omitempty"`
+}
+
+// StreamError 流式错误
+type StreamError struct {
+	Type    string `json:"type"`
+	Message string `json:"message"`
+}
+
+// NewMessageStartEvent 创建消息开始事件
+func NewMessageStartEvent(id, model string) CanonicalStreamEvent {
+	return CanonicalStreamEvent{
+		Type:    EventMessageStart,
+		Message: &StreamMessage{ID: id, Model: model},
+	}
+}
+
+// NewMessageStartEventWithUsage 创建带用量的消息开始事件
+func NewMessageStartEventWithUsage(id, model string, usage *CanonicalUsage) CanonicalStreamEvent {
+	return CanonicalStreamEvent{
+		Type:    EventMessageStart,
+		Message: &StreamMessage{ID: id, Model: model, Usage: usage},
+	}
+}
+
+// NewContentBlockStartEvent 创建内容块开始事件
+func NewContentBlockStartEvent(index int, block StreamContentBlock) CanonicalStreamEvent {
+	idx := index
+	return CanonicalStreamEvent{
+		Type:         EventContentBlockStart,
+		Index:        &idx,
+		ContentBlock: &block,
+	}
+}
+
+// NewContentBlockDeltaEvent 创建内容块增量事件
+func NewContentBlockDeltaEvent(index int, delta StreamDelta) CanonicalStreamEvent {
+	idx := index
+	return CanonicalStreamEvent{
+		Type:  EventContentBlockDelta,
+		Index: &idx,
+		Delta: &delta,
+	}
+}
+
+// NewContentBlockStopEvent 创建内容块结束事件
+func NewContentBlockStopEvent(index int) CanonicalStreamEvent {
+	idx := index
+	return CanonicalStreamEvent{
+		Type:  EventContentBlockStop,
+		Index: &idx,
+	}
+}
+
+// NewMessageDeltaEvent 创建消息增量事件
+func NewMessageDeltaEvent(stopReason StopReason) CanonicalStreamEvent {
+	sr := stopReason
+	return CanonicalStreamEvent{
+		Type:       EventMessageDelta,
+		StopReason: &sr,
+	}
+}
+
+// NewMessageDeltaEventWithUsage 创建带用量的消息增量事件
+func NewMessageDeltaEventWithUsage(stopReason StopReason, usage *CanonicalUsage) CanonicalStreamEvent {
+	sr := stopReason
+	return CanonicalStreamEvent{
+		Type:       EventMessageDelta,
+		StopReason: &sr,
+		Usage:      usage,
+	}
+}
+
+// NewMessageStopEvent 创建消息结束事件
+func NewMessageStopEvent() CanonicalStreamEvent {
+	return CanonicalStreamEvent{Type: EventMessageStop}
+}
+
+// NewErrorEvent 创建错误事件
+func NewErrorEvent(errType, message string) CanonicalStreamEvent {
+	return CanonicalStreamEvent{
+		Type:  EventError,
+		Error: &StreamError{Type: errType, Message: message},
+	}
+}
+
+// NewPingEvent 创建心跳事件
+func NewPingEvent() CanonicalStreamEvent {
+	return CanonicalStreamEvent{Type: EventPing}
+}