Extract BaseSession request ID increment logic

This allows the request ID increment logic to be overwritten by
subclasses. I have a unique need to share sessions' underlying input
and output streams, but not the sessions themselves. This would require
keeping track of request IDs globally to avoid overlap.

Having this logic overridable lets me do that.

Author: Ron Mordechai

src/mcp/shared/session.py

@@ -233,8 +233,7 @@ async def send_request(
         Do not use this method to emit notifications! Use send_notification()
         instead.
         """
-        request_id = self._request_id
-        self._request_id = request_id + 1
+        request_id = self.next_request_id()
 
         response_stream, response_stream_reader = anyio.create_memory_object_stream[JSONRPCResponse | JSONRPCError](1)
         self._response_streams[request_id] = response_stream
@@ -468,3 +467,12 @@ async def _handle_incoming(
     ) -> None:
         """A generic handler for incoming messages. Overwritten by subclasses."""
         pass
+
+    def next_request_id(self) -> int:
+        """
+        Increment the request ID and return its current value. Can be
+        overwritten by subclasses.
+        """
+        request_id = self._request_id
+        self._request_id = request_id + 1
+        return request_id