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/anthropic/stream_decoder_test.go

@@ -272,3 +272,218 @@ func TestStreamDecoder_RedactedDeltaSuppressed(t *testing.T) {
 	events := d.ProcessChunk(raw)
 	assert.Empty(t, events)
 }
+
+func TestStreamDecoder_ServerToolUse_Suppressed(t *testing.T) {
+	d := NewStreamDecoder()
+
+	payload := map[string]any{
+		"type":  "content_block_start",
+		"index": 2,
+		"content_block": map[string]any{
+			"type": "server_tool_use",
+			"id":   "server_tool_1",
+			"name": "web_search",
+		},
+	}
+	raw := makeAnthropicEvent("content_block_start", payload)
+	events := d.ProcessChunk(raw)
+	assert.Empty(t, events)
+	assert.True(t, d.redactedBlocks[2])
+}
+
+func TestStreamDecoder_WebSearchToolResult_Suppressed(t *testing.T) {
+	d := NewStreamDecoder()
+
+	payload := map[string]any{
+		"type":  "content_block_start",
+		"index": 3,
+		"content_block": map[string]any{
+			"type": "web_search_tool_result",
+			"tool_use_id": "search_1",
+		},
+	}
+	raw := makeAnthropicEvent("content_block_start", payload)
+	events := d.ProcessChunk(raw)
+	assert.Empty(t, events)
+	assert.True(t, d.redactedBlocks[3])
+}
+
+func TestStreamDecoder_CodeExecutionToolResult_Suppressed(t *testing.T) {
+	d := NewStreamDecoder()
+
+	payload := map[string]any{
+		"type":  "content_block_start",
+		"index": 4,
+		"content_block": map[string]any{
+			"type": "code_execution_tool_result",
+		},
+	}
+	raw := makeAnthropicEvent("content_block_start", payload)
+	events := d.ProcessChunk(raw)
+	assert.Empty(t, events)
+	assert.True(t, d.redactedBlocks[4])
+}
+
+func TestStreamDecoder_CitationsDelta_Discarded(t *testing.T) {
+	d := NewStreamDecoder()
+
+	payload := map[string]any{
+		"type":  "content_block_delta",
+		"index": 0,
+		"delta": map[string]any{
+			"type":        "citations_delta",
+			"citation":    map[string]any{"title": "ref1"},
+		},
+	}
+	raw := makeAnthropicEvent("content_block_delta", payload)
+	events := d.ProcessChunk(raw)
+	assert.Empty(t, events)
+}
+
+func TestStreamDecoder_SignatureDelta_Discarded(t *testing.T) {
+	d := NewStreamDecoder()
+
+	payload := map[string]any{
+		"type":  "content_block_delta",
+		"index": 0,
+		"delta": map[string]any{
+			"type":      "signature_delta",
+			"signature": "sig_123",
+		},
+	}
+	raw := makeAnthropicEvent("content_block_delta", payload)
+	events := d.ProcessChunk(raw)
+	assert.Empty(t, events)
+}
+
+func TestStreamDecoder_UnknownEventType(t *testing.T) {
+	d := NewStreamDecoder()
+
+	raw := makeAnthropicEvent("unknown_event", map[string]any{"type": "unknown_event"})
+	events := d.ProcessChunk(raw)
+	assert.Empty(t, events)
+}
+
+func TestStreamDecoder_InvalidJSON(t *testing.T) {
+	d := NewStreamDecoder()
+
+	raw := []byte("event: message_start\ndata: {invalid}\n\n")
+	events := d.ProcessChunk(raw)
+	assert.Empty(t, events)
+}
+
+func TestStreamDecoder_MultipleEventsInSingleChunk(t *testing.T) {
+	d := NewStreamDecoder()
+
+	startPayload := map[string]any{
+		"type": "message_start",
+		"message": map[string]any{
+			"id":    "msg_multi",
+			"model": "claude-3",
+		},
+	}
+	deltaPayload := map[string]any{
+		"type":  "content_block_delta",
+		"index": 0,
+		"delta": map[string]any{
+			"type": "text_delta",
+			"text": "Hello",
+		},
+	}
+	stopPayload := map[string]any{"type": "message_stop"}
+
+	var raw []byte
+	raw = append(raw, makeAnthropicEvent("message_start", startPayload)...)
+	raw = append(raw, makeAnthropicEvent("content_block_delta", deltaPayload)...)
+	raw = append(raw, makeAnthropicEvent("message_stop", stopPayload)...)
+
+	events := d.ProcessChunk(raw)
+	require.Len(t, events, 3)
+	assert.Equal(t, canonical.EventMessageStart, events[0].Type)
+	assert.Equal(t, canonical.EventContentBlockDelta, events[1].Type)
+	assert.Equal(t, canonical.EventMessageStop, events[2].Type)
+}
+
+func TestStreamDecoder_ErrorInvalidJSON(t *testing.T) {
+	d := NewStreamDecoder()
+
+	raw := []byte("event: error\ndata: {invalid}\n\n")
+	events := d.ProcessChunk(raw)
+	require.Len(t, events, 1)
+	assert.Equal(t, canonical.EventError, events[0].Type)
+	assert.Contains(t, events[0].Error.Message, "解析错误事件失败")
+}
+
+func TestStreamDecoder_MessageStartWithUsage(t *testing.T) {
+	d := NewStreamDecoder()
+
+	payload := map[string]any{
+		"type": "message_start",
+		"message": map[string]any{
+			"id":    "msg_usage",
+			"model": "claude-3",
+			"usage": map[string]any{"input_tokens": 25, "output_tokens": 0},
+		},
+	}
+	raw := makeAnthropicEvent("message_start", payload)
+
+	events := d.ProcessChunk(raw)
+	require.Len(t, events, 1)
+	assert.Equal(t, canonical.EventMessageStart, events[0].Type)
+	require.NotNil(t, events[0].Message.Usage)
+	assert.Equal(t, 25, events[0].Message.Usage.InputTokens)
+}
+
+func TestStreamDecoder_ThinkingBlockStart(t *testing.T) {
+	d := NewStreamDecoder()
+
+	payload := map[string]any{
+		"type":  "content_block_start",
+		"index": 0,
+		"content_block": map[string]any{
+			"type":     "thinking",
+			"thinking": "",
+		},
+	}
+	raw := makeAnthropicEvent("content_block_start", payload)
+
+	events := d.ProcessChunk(raw)
+	require.Len(t, events, 1)
+	assert.Equal(t, canonical.EventContentBlockStart, events[0].Type)
+	require.NotNil(t, events[0].ContentBlock)
+	assert.Equal(t, "thinking", events[0].ContentBlock.Type)
+}
+
+func TestStreamDecoder_MessageDelta_UsageNotAccumulated(t *testing.T) {
+	d := NewStreamDecoder()
+
+	startPayload := map[string]any{
+		"type": "message_start",
+		"message": map[string]any{
+			"id":    "msg_usage_test",
+			"model": "claude-3",
+			"usage": map[string]any{"input_tokens": 10, "output_tokens": 0},
+		},
+	}
+	deltaPayload1 := map[string]any{
+		"type": "message_delta",
+		"delta": map[string]any{"stop_reason": "end_turn"},
+		"usage": map[string]any{"output_tokens": 25},
+	}
+
+	d.ProcessChunk(makeAnthropicEvent("message_start", startPayload))
+	events := d.ProcessChunk(makeAnthropicEvent("message_delta", deltaPayload1))
+
+	require.Len(t, events, 1)
+	assert.Equal(t, 25, events[0].Usage.OutputTokens)
+
+	deltaPayload2 := map[string]any{
+		"type": "message_delta",
+		"delta": map[string]any{"stop_reason": "end_turn"},
+		"usage": map[string]any{"output_tokens": 30},
+	}
+	events = d.ProcessChunk(makeAnthropicEvent("message_delta", deltaPayload2))
+	require.Len(t, events, 1)
+	assert.Equal(t, 30, events[0].Usage.OutputTokens, "output_tokens should be replaced, not accumulated")
+	assert.Equal(t, 30, d.accumulatedUsage.OutputTokens, "accumulated usage should match last value")
+}