feat(V2): ClientRegistry lazy con lock per chiave

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/cerbero_mcp/client_registry.py

@@ -0,0 +1,41 @@
+"""Cache lazy di client exchange, una istanza per (exchange, env)."""
+from __future__ import annotations
+
+import asyncio
+from collections import defaultdict
+from collections.abc import Awaitable, Callable
+from typing import Any, Literal
+
+Environment = Literal["testnet", "mainnet"]
+Builder = Callable[[str, Environment], Awaitable[Any]]
+
+
+class ClientRegistry:
+    def __init__(self, *, builder: Builder) -> None:
+        self._builder = builder
+        self._clients: dict[tuple[str, Environment], Any] = {}
+        self._locks: dict[tuple[str, Environment], asyncio.Lock] = defaultdict(
+            asyncio.Lock
+        )
+
+    async def get(self, exchange: str, env: Environment) -> Any:
+        key = (exchange, env)
+        if key in self._clients:
+            return self._clients[key]
+        async with self._locks[key]:
+            if key in self._clients:  # double-check
+                return self._clients[key]
+            client = await self._builder(exchange, env)
+            self._clients[key] = client
+            return client
+
+    async def aclose(self) -> None:
+        for client in self._clients.values():
+            close = getattr(client, "aclose", None)
+            if close is None:
+                continue
+            try:
+                await close()
+            except Exception:
+                pass
+        self._clients.clear()