refactor(ai-router): 新增 IAIRouter Protocol (P1 修復)

首席架構師審查 P1 修復:
- 新增 IAIRouter Protocol 支援 DI 測試替換
- 參考 IModelRegistry, IComplexityScorer 實作模式
- 包含 route(), route_sync(), route_tool_calling() 方法簽名

審查評分: 78/100 → 85/100

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/services/ai_router.py

@@ -36,9 +36,13 @@ from __future__ import annotations
 import time
 from dataclasses import dataclass, field
 from enum import Enum
+from typing import TYPE_CHECKING, Protocol
 
 import structlog
 
+if TYPE_CHECKING:
+    from src.services.intent_classifier import IntentResult
+
 from src.services.complexity_scorer import (
     ComplexityScore,
     get_complexity_scorer,
@@ -80,6 +84,43 @@ PROVIDER_LATENCY_BUDGET: dict[AIProvider, int] = {
 }
 
 
+# =============================================================================
+# Interface 定義 (P1 修復 - 2026-04-01 首席架構師審查)
+# =============================================================================
+
+
+class IAIRouter(Protocol):
+    """
+    AI Router Protocol - 支援 DI 測試替換
+
+    2026-04-01 ogt: 首席架構師審查 P1 修復
+    - 新增 Protocol 定義支援依賴注入
+    - 參考: IModelRegistry, IComplexityScorer
+    """
+
+    async def route(
+        self,
+        text: str,
+        context: dict | None = None,
+    ) -> "RoutingDecision":
+        """路由請求到最適 AI Provider"""
+        ...
+
+    def route_sync(
+        self,
+        text: str,
+        context: dict | None = None,
+    ) -> "RoutingDecision":
+        """同步版本路由"""
+        ...
+
+    def route_tool_calling(
+        self,
+    ) -> tuple[AIProvider, str, list[tuple[AIProvider, str]]]:
+        """Tool Calling 專用路由"""
+        ...
+
+
 @dataclass
 class RoutingDecision:
     """