Fix idle timer not reset after successful GCS Read completion

The idle timer was started/reset before each Read call but never reset
after successful completion. This meant the effective inter-Read idle
budget was reduced by the read duration itself. If a single GCS Read
took 9s, only 1s remained for the consumer before the timer fired.

A slow consumer (UFFD back-pressure, decompressor buffering) could
easily exceed that shrunken window, causing spurious context
cancellation - exactly the scenario the per-Read idle timer was meant
to protect against.

Adding timer.Reset after successful reads gives the consumer the full
idle budget after each completed read operation.

Author: cursoragent

packages/shared/pkg/storage/storage_google.go

@@ -303,6 +303,8 @@ func (r *idleTimeoutReader) Read(p []byte) (int, error) {
 	n, err := r.ReadCloser.Read(p)
 	if err != nil {
 		r.timer.Stop()
+	} else {
+		r.timer.Reset(r.idle)
 	}
 	return n, err
 }