chore(api): fix import error in `base_session.py`, add some comments

api/core/mcp/session/base_session.py

@@ -1,7 +1,7 @@
 import logging
 import queue
 from collections.abc import Callable
-from concurrent.futures import Future, ThreadPoolExecutor
+from concurrent.futures import Future, ThreadPoolExecutor, TimeoutError
 from contextlib import ExitStack
 from datetime import timedelta
 from types import TracebackType
@@ -176,11 +176,15 @@ class BaseSession(
         self._receiver_future: Future | None = None
 
     def __enter__(self) -> Self:
-        self._executor = ThreadPoolExecutor()
+        # The thread pool is dedicated to running `_receive_loop`. Setting `max_workers` to 1
+        # ensures no unnecessary threads are created.
+        self._executor = ThreadPoolExecutor(max_workers=1)
         self._receiver_future = self._executor.submit(self._receive_loop)
         return self
 
     def check_receiver_status(self) -> None:
+        """`check_receiver_status` ensures that any exceptions raised during the
+        execution of `_receive_loop` are retrieved and propagated."""
         if self._receiver_future and self._receiver_future.done():
             self._receiver_future.result()
 
@@ -194,7 +198,7 @@ class BaseSession(
         if self._receiver_future:
             try:
                 self._receiver_future.result(timeout=5.0)  # Wait up to 5 seconds
-            except concurrent.futures.TimeoutError:
+            except TimeoutError:
                 # If the receiver loop is still running after timeout, we'll force shutdown
                 pass