Preserve sessions index page when navigating back

What does this MR do and why?

When a user navigates from the sessions index to a session detail page and then back, the index component is unmounted and remounted, losing the pagination variables. The Apollo cache may still hold the data, but the query is re-issued with the default (first-page) variables, so the user lands on page 1 instead of the page they were on.

Fix: add a lightweight sessions_query_state.js module that stores the last-used query variables (sort, pagination, filters) in a module-level variable. Both UserAgentsPlatformIndex and ProjectAgentsPlatformIndex read the stored variables on mount and write them whenever the child emits query-variables-updated. Because the module is not part of the navigation_state.js import chain, it avoids the circular dependency that would otherwise arise.

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/571496

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Navigate to the agent sessions index page (e.g. /group/-/agent-sessions).
2. Page forward to page 2 or apply a filter/sort.
3. Click into a session detail.
4. Click the browser back button or the breadcrumb to return to the index.
5. Before: lands on page 1 with default sort. After: lands on the same page/sort/filter as before.

MR acceptance checklist

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