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

- 新增 ConversionEngine 核心引擎，支持 OpenAI 和 Anthropic 协议转换
- 添加 stream decoder/encoder 实现
- 更新 provider client 支持新引擎
- 补充单元测试和集成测试
- 更新 specs 文档

backend/internal/conversion/stream_test.go

@@ -1,6 +1,7 @@
 package conversion
 
 import (
+	"fmt"
 	"testing"
 
 	"nex/backend/internal/conversion/canonical"
@@ -128,3 +129,71 @@ func TestCanonicalStreamConverter_EmptyDecoder(t *testing.T) {
 
 	assert.Nil(t, result)
 }
+
+func TestCanonicalStreamConverter_MiddlewareError_Continue(t *testing.T) {
+	event := canonical.NewMessageStartEvent("id-1", "gpt-4")
+	decoder := &mockStreamDecoder{
+		chunks: [][]canonical.CanonicalStreamEvent{{event}},
+	}
+	encoder := &mockStreamEncoder{
+		events: [][]byte{[]byte("data: ok\n\n")},
+	}
+
+	chain := NewMiddlewareChain()
+	chain.Use(&errorMiddleware{})
+	ctx := NewConversionContext(InterfaceTypeChat)
+
+	converter := NewCanonicalStreamConverterWithMiddleware(decoder, encoder, chain, *ctx, "openai", "anthropic")
+	result := converter.ProcessChunk([]byte("raw"))
+
+	assert.Nil(t, result, "middleware error should cause the event to be skipped (continue)")
+}
+
+func TestCanonicalStreamConverter_Flush_MiddlewareError_Continue(t *testing.T) {
+	event := canonical.NewMessageStartEvent("id-1", "gpt-4")
+	decoder := &mockStreamDecoder{
+		flush: []canonical.CanonicalStreamEvent{event},
+	}
+	encoder := &mockStreamEncoder{
+		events: [][]byte{[]byte("data: ok\n\n")},
+		flush:  [][]byte{[]byte("data: encoder_flush\n\n")},
+	}
+
+	chain := NewMiddlewareChain()
+	chain.Use(&errorMiddleware{})
+	ctx := NewConversionContext(InterfaceTypeChat)
+
+	converter := NewCanonicalStreamConverterWithMiddleware(decoder, encoder, chain, *ctx, "openai", "anthropic")
+	result := converter.Flush()
+
+	assert.Len(t, result, 1)
+	assert.Equal(t, []byte("data: encoder_flush\n\n"), result[0])
+}
+
+func TestCanonicalStreamConverter_Flush_DecoderAndEncoderBothProduce(t *testing.T) {
+	event := canonical.NewMessageStartEvent("id-1", "gpt-4")
+	decoder := &mockStreamDecoder{
+		flush: []canonical.CanonicalStreamEvent{event},
+	}
+	encoder := &mockStreamEncoder{
+		events: [][]byte{[]byte("data: decoder_flush\n\n")},
+		flush:  [][]byte{[]byte("data: encoder_flush\n\n")},
+	}
+
+	converter := NewCanonicalStreamConverter(decoder, encoder)
+	result := converter.Flush()
+
+	assert.Len(t, result, 2)
+	assert.Equal(t, []byte("data: decoder_flush\n\n"), result[0])
+	assert.Equal(t, []byte("data: encoder_flush\n\n"), result[1])
+}
+
+type errorMiddleware struct{}
+
+func (m *errorMiddleware) Intercept(req *canonical.CanonicalRequest, clientProtocol, providerProtocol string, ctx *ConversionContext) (*canonical.CanonicalRequest, error) {
+	return nil, fmt.Errorf("middleware error")
+}
+
+func (m *errorMiddleware) InterceptStreamEvent(event *canonical.CanonicalStreamEvent, clientProtocol, providerProtocol string, ctx *ConversionContext) (*canonical.CanonicalStreamEvent, error) {
+	return nil, fmt.Errorf("stream middleware error")
+}