fix: make pool initialize() idempotent to prevent double init

The pattern `async with await create_pool(...)` called initialize()
twice (once in create_pool, once in __aenter__), creating 2x min_size
connections. Now guarded by an _initialized flag.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqliteclient/pool.py

@@ -44,9 +44,13 @@ def __init__(
         self._size = 0
         self._lock = asyncio.Lock()
         self._closed = False
+        self._initialized = False
 
     async def initialize(self) -> None:
         """Initialize the pool with minimum connections."""
+        if self._initialized:
+            return
+        self._initialized = True
         for _ in range(self._min_size):
             conn = await self._create_connection()
             await self._pool.put(conn)

tests/test_pool.py

@@ -32,6 +32,30 @@ async def test_acquire_when_closed(self) -> None:
             async with pool.acquire():
                 pass
 
+    async def test_double_initialize_is_idempotent(self) -> None:
+        """Calling initialize() twice should not create double connections."""
+        pool = ConnectionPool(["localhost:9001"], min_size=2, max_size=5)
+
+        mock_conn = MagicMock()
+        mock_conn.is_connected = True
+        mock_conn.connect = AsyncMock()
+        mock_conn.close = AsyncMock()
+
+        create_count = 0
+
+        async def mock_connect(**kwargs):
+            nonlocal create_count
+            create_count += 1
+            return mock_conn
+
+        with patch.object(pool._cluster, "connect", side_effect=mock_connect):
+            await pool.initialize()
+            await pool.initialize()  # Second call should be no-op
+
+        assert create_count == 2  # Only 2 (min_size), not 4
+
+        await pool.close()
+
     async def test_cancellation_does_not_leak_connection(self) -> None:
         """Cancelling a task that holds a connection should clean it up."""
         import asyncio