perf: use deque for InMemoryTaskMessageQueue FIFO operations

Replace list with collections.deque in InMemoryTaskMessageQueue so
that dequeue (popleft) runs in O(1) amortised time instead of the
O(n) incurred by list.pop(0), which must shift every remaining
element on each call.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: g97iulio1609

src/mcp/shared/experimental/tasks/message_queue.py

@@ -12,6 +12,7 @@
 """
 
 from abc import ABC, abstractmethod
+from collections import deque
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from typing import Any, Literal
@@ -151,13 +152,13 @@ class InMemoryTaskMessageQueue(TaskMessageQueue):
     """
 
     def __init__(self) -> None:
-        self._queues: dict[str, list[QueuedMessage]] = {}
+        self._queues: dict[str, deque[QueuedMessage]] = {}
         self._events: dict[str, anyio.Event] = {}
 
-    def _get_queue(self, task_id: str) -> list[QueuedMessage]:
+    def _get_queue(self, task_id: str) -> deque[QueuedMessage]:
         """Get or create the queue for a task."""
         if task_id not in self._queues:
-            self._queues[task_id] = []
+            self._queues[task_id] = deque()
         return self._queues[task_id]
 
     async def enqueue(self, task_id: str, message: QueuedMessage) -> None:
@@ -172,7 +173,7 @@ async def dequeue(self, task_id: str) -> QueuedMessage | None:
         queue = self._get_queue(task_id)
         if not queue:
             return None
-        return queue.pop(0)
+        return queue.popleft()
 
     async def peek(self, task_id: str) -> QueuedMessage | None:
         """Return the next message without removing it."""