Improve fallback mechanism when the MR list times out

Summary

When the MR list query times out on the MR list page, users are currently shown outdated/stale data from previous successful queries.

This creates confusion especially if the last successful query ran on a different tab. E.g. users may see the list of open MRs under the Merged tab.

The underlying problem is the timeout, which should be fixed in #563367, but in the meantime we can explore improving the flabback situation to reduce confusion.

Report in Slack: https://gitlab.slack.com/archives/C01EMBKS5DW/p1758286010225469 (internal)

the "Merged" tab for gitlab-com seems to time out, and then incorrectly displays open merge requests

Proposal

When listing MRs times out:

1. Do not display MRs from previous successful queries.
   1. Question: or maybe do, but only if they are for the same tab?
2. Show an error message to suggest using filters, as this is a typical workaround to get results when there are too many MRs in a group/project.