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/openai/stream_encoder_test.go

@@ -170,3 +170,116 @@ func TestStreamEncoder_MessageDelta_WithUsage(t *testing.T) {
 	assert.Contains(t, s, "usage")
 	assert.Contains(t, s, "prompt_tokens")
 }
+
+func TestStreamEncoder_InputJSONDelta_SubsequentDelta(t *testing.T) {
+	e := NewStreamEncoder()
+
+	e.EncodeEvent(canonical.NewContentBlockStartEvent(0, canonical.StreamContentBlock{
+		Type: "tool_use",
+		ID:   "call_1",
+		Name: "get_weather",
+	}))
+
+	e.EncodeEvent(canonical.NewContentBlockDeltaEvent(0, canonical.StreamDelta{
+		Type:        string(canonical.DeltaTypeInputJSON),
+		PartialJSON: "{\"city\":",
+	}))
+
+	event := canonical.NewContentBlockDeltaEvent(0, canonical.StreamDelta{
+		Type:        string(canonical.DeltaTypeInputJSON),
+		PartialJSON: "\"Beijing\"}",
+	})
+
+	chunks := e.EncodeEvent(event)
+	require.NotEmpty(t, chunks)
+
+	s := string(chunks[0])
+	assert.Contains(t, s, "tool_calls")
+	assert.Contains(t, s, "Beijing")
+}
+
+func TestStreamEncoder_MessageStart_NilMessage(t *testing.T) {
+	e := NewStreamEncoder()
+	event := canonical.CanonicalStreamEvent{Type: canonical.EventMessageStart}
+
+	chunks := e.EncodeEvent(event)
+	require.Len(t, chunks, 1)
+
+	s := string(chunks[0])
+	assert.Contains(t, s, "chat.completion.chunk")
+}
+
+func TestStreamEncoder_UnknownEvent_ReturnsNil(t *testing.T) {
+	e := NewStreamEncoder()
+	event := canonical.CanonicalStreamEvent{Type: "unknown_type"}
+	chunks := e.EncodeEvent(event)
+	assert.Nil(t, chunks)
+}
+
+func TestStreamEncoder_ContentBlockDelta_NilDelta(t *testing.T) {
+	e := NewStreamEncoder()
+	event := canonical.CanonicalStreamEvent{Type: canonical.EventContentBlockDelta}
+	chunks := e.EncodeEvent(event)
+	assert.Nil(t, chunks)
+}
+
+func TestStreamEncoder_MultiToolCall_IndexMapping(t *testing.T) {
+	e := NewStreamEncoder()
+
+	e.EncodeEvent(canonical.NewContentBlockStartEvent(0, canonical.StreamContentBlock{
+		Type: "tool_use",
+		ID:   "call_1",
+		Name: "get_weather",
+	}))
+
+	firstDelta := canonical.NewContentBlockDeltaEvent(0, canonical.StreamDelta{
+		Type:        string(canonical.DeltaTypeInputJSON),
+		PartialJSON: `{"city":"北京"}`,
+	})
+	chunks := e.EncodeEvent(firstDelta)
+	require.NotEmpty(t, chunks)
+	s := string(chunks[0])
+	assert.Contains(t, s, `"index":0`)
+	assert.Contains(t, s, "get_weather")
+	assert.Contains(t, s, "北京")
+
+	e.EncodeEvent(canonical.NewContentBlockStopEvent(0))
+
+	e.EncodeEvent(canonical.NewContentBlockStartEvent(1, canonical.StreamContentBlock{
+		Type: "tool_use",
+		ID:   "call_2",
+		Name: "get_time",
+	}))
+
+	secondDelta := canonical.NewContentBlockDeltaEvent(1, canonical.StreamDelta{
+		Type:        string(canonical.DeltaTypeInputJSON),
+		PartialJSON: `{"tz":"Asia/Shanghai"}`,
+	})
+	chunks = e.EncodeEvent(secondDelta)
+	require.NotEmpty(t, chunks)
+	s = string(chunks[0])
+	assert.Contains(t, s, `"index":1`)
+	assert.Contains(t, s, "get_time")
+	assert.Contains(t, s, "Asia/Shanghai")
+
+	subsequentDelta0 := canonical.NewContentBlockDeltaEvent(0, canonical.StreamDelta{
+		Type:        string(canonical.DeltaTypeInputJSON),
+		PartialJSON: `"more_data"`,
+	})
+	chunks = e.EncodeEvent(subsequentDelta0)
+	require.NotEmpty(t, chunks)
+	s = string(chunks[0])
+	assert.Contains(t, s, `"index":0`)
+	assert.NotContains(t, s, "get_weather")
+	assert.Contains(t, s, "more_data")
+
+	subsequentDelta1 := canonical.NewContentBlockDeltaEvent(1, canonical.StreamDelta{
+		Type:        string(canonical.DeltaTypeInputJSON),
+		PartialJSON: `"more_time"`,
+	})
+	chunks = e.EncodeEvent(subsequentDelta1)
+	require.NotEmpty(t, chunks)
+	s = string(chunks[0])
+	assert.Contains(t, s, `"index":1`)
+	assert.Contains(t, s, "more_time")
+}