fix(page_service): Timeline::gate held open while throttling (#10314)

When we moved throttling up from Timeline::get into page_service,
we stopped being sensitive to `Timeline::cancel`, even though we're
holding a Handle and thus a guard on the `Timeline::gate` open.

This PR rectifies the situation.

Refs

- Found while investigating #10309 (hung detach because gate kept open),
  but not expected to be the root cause of that issue because the
  affected tenants are not being throttled according to their metrics.

pageserver/src/page_service.rs

@@ -618,6 +618,9 @@ impl BatchedFeMessage {
         };
         let throttled = tokio::select! {
             throttled = shard.pagestream_throttle.throttle(tokens) => { throttled }
+            _ = shard.cancel.cancelled() => {
+                return Err(QueryError::Shutdown);
+            }
             _ = cancel.cancelled() => {
                 return Err(QueryError::Shutdown);
             }