feat(proxy): T-P2 上游客户端（流式派发/usage 注入过滤/三家归一化/首 token 前 failover）

- upstream.py：httpx.AsyncClient 模块级单例（keepalive，limits=100，
  超时 connect10/read120/write10/pool30）；stream() 始终注入
  stream_options.include_usage（计量不依赖客户端）+ filter_usage_chunk
  （客户端未要求 usage 时剥除该 chunk）；D-P4 首 token 前 failover 链、
  流中失败抛 UpstreamAborted（不可切换）；ttfb_ms 记录
- normalize_usage 三家归一：deepseek prompt_cache_hit_tokens /
  openai prompt_tokens_details.cached_tokens / anthropic cache_read_input_tokens
  （命中数>总数时钳制）
- model_pool：+provider（枚举校验 deepseek/openai/anthropic）+in_hit_price
  （缺省 = price_in×1/30，D-P3）
- 测试 +6：透传+sink/failover/流中 aborted/全挂 502/三家归一/过滤，全量 345 passed

gateway/model_pool.py

@@ -21,11 +21,13 @@ _POOL_PATH = Path(__file__).resolve().parent.parent / "config" / "model_pool.jso
 TIERS = ("local", "budget", "premium")
 BACKENDS = ("mock", "llama_server", "openai")
 ROLES = ("architect", "worker", "agent")
+PROVIDERS = ("deepseek", "openai", "anthropic")   # 代理层 usage 归一化用（D-P3）
 
 # 池条目允许的字段（其余字段拒绝写入）
 ENTRY_FIELDS = {
     "id", "name", "tier", "backend", "base_url", "model", "api_key",
     "price_in", "price_out", "temperature", "max_tokens", "enabled",
+    "provider", "in_hit_price",   # 代理层扩展（D-P3）：usage 归一化 / 按命中价选上游
 }
 
 # 单价默认值（$/1M tokens）；local 档为 0
@@ -177,6 +179,8 @@ class PoolStore:
         backend = entry.get("backend", "openai")
         if backend not in BACKENDS:
             raise PoolError(f"backend 必须是 {BACKENDS} 之一")
+        if entry.get("provider") and entry["provider"] not in PROVIDERS:
+            raise PoolError(f"provider 必须是 {PROVIDERS} 之一")
         tier = entry.get("tier", "budget")
         if tier not in TIERS:
             raise PoolError(f"tier 必须是 {TIERS} 之一")
@@ -212,6 +216,10 @@ class PoolStore:
             "temperature": temperature,
             "max_tokens": max_tokens,
             "enabled": bool(entry.get("enabled", True)),
+            # 代理层扩展（D-P3）：provider 缺省 openai；in_hit_price 缺省 = price_in × 1/30
+            "provider": (str(entry["provider"]) if entry.get("provider") else "openai"),
+            "in_hit_price": (float(entry["in_hit_price"]) if entry.get("in_hit_price") is not None
+                             else round(price_in / 30.0, 6)),
         }
 
     @staticmethod