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/stream.go

@@ -0,0 +1,107 @@
+package conversion
+
+import "nex/backend/internal/conversion/canonical"
+
+// StreamDecoder 流式解码器接口
+type StreamDecoder interface {
+	ProcessChunk(rawChunk []byte) []canonical.CanonicalStreamEvent
+	Flush() []canonical.CanonicalStreamEvent
+}
+
+// StreamEncoder 流式编码器接口
+type StreamEncoder interface {
+	EncodeEvent(event canonical.CanonicalStreamEvent) [][]byte
+	Flush() [][]byte
+}
+
+// StreamConverter 流式转换器接口
+type StreamConverter interface {
+	ProcessChunk(rawChunk []byte) [][]byte
+	Flush() [][]byte
+}
+
+// PassthroughStreamConverter 同协议透传流式转换器
+type PassthroughStreamConverter struct{}
+
+// NewPassthroughStreamConverter 创建透传流式转换器
+func NewPassthroughStreamConverter() *PassthroughStreamConverter {
+	return &PassthroughStreamConverter{}
+}
+
+// ProcessChunk 直接传递原始字节
+func (c *PassthroughStreamConverter) ProcessChunk(rawChunk []byte) [][]byte {
+	return [][]byte{rawChunk}
+}
+
+// Flush 无缓冲数据
+func (c *PassthroughStreamConverter) Flush() [][]byte {
+	return nil
+}
+
+// CanonicalStreamConverter 跨协议规范流式转换器
+type CanonicalStreamConverter struct {
+	decoder  StreamDecoder
+	encoder  StreamEncoder
+	chain    *MiddlewareChain
+	ctx      ConversionContext
+	clientProtocol  string
+	providerProtocol string
+}
+
+// NewCanonicalStreamConverter 创建规范流式转换器
+func NewCanonicalStreamConverter(decoder StreamDecoder, encoder StreamEncoder) *CanonicalStreamConverter {
+	return &CanonicalStreamConverter{
+		decoder: decoder,
+		encoder: encoder,
+	}
+}
+
+// NewCanonicalStreamConverterWithMiddleware 创建带中间件的规范流式转换器
+func NewCanonicalStreamConverterWithMiddleware(decoder StreamDecoder, encoder StreamEncoder, chain *MiddlewareChain, ctx ConversionContext, clientProtocol, providerProtocol string) *CanonicalStreamConverter {
+	return &CanonicalStreamConverter{
+		decoder:         decoder,
+		encoder:         encoder,
+		chain:           chain,
+		ctx:             ctx,
+		clientProtocol:  clientProtocol,
+		providerProtocol: providerProtocol,
+	}
+}
+
+// ProcessChunk 解码 → 中间件 → 编码管道
+func (c *CanonicalStreamConverter) ProcessChunk(rawChunk []byte) [][]byte {
+	events := c.decoder.ProcessChunk(rawChunk)
+	var result [][]byte
+	for i := range events {
+		if c.chain != nil {
+			processed, err := c.chain.ApplyStreamEvent(&events[i], c.clientProtocol, c.providerProtocol, &c.ctx)
+			if err != nil {
+				continue
+			}
+			events[i] = *processed
+		}
+		chunks := c.encoder.EncodeEvent(events[i])
+		result = append(result, chunks...)
+	}
+	return result
+}
+
+// Flush 刷新解码器和编码器缓冲区
+func (c *CanonicalStreamConverter) Flush() [][]byte {
+	events := c.decoder.Flush()
+	var result [][]byte
+	for i := range events {
+		if c.chain != nil {
+			processed, err := c.chain.ApplyStreamEvent(&events[i], c.clientProtocol, c.providerProtocol, &c.ctx)
+			if err != nil {
+				continue
+			}
+			events[i] = *processed
+		}
+		chunks := c.encoder.EncodeEvent(events[i])
+		result = append(result, chunks...)
+	}
+	encoderChunks := c.encoder.Flush()
+	result = append(result, encoderChunks...)
+	return result
+}