Hide time-range filter in Duo panel session inbox

What does this MR do and why?

The Duo panel sidebar inbox (agent_session_inbox.vue) rendered the same "All" / "Available" time-range dropdown as the full-page Automate > Sessions index, via the shared agent_flow_filtered_search.vue component. The panel is a "jump back in" view: per UX it should only ever show sessions still within the 30-day retention window, so the dropdown is noise there. This MR drops it from the panel, scopes the panel's query to available sessions, and leaves the full-page index untouched.

agent_flow_filtered_search.vue gets a showTimeRangeFilter prop (default true) that gates the #time-range-filter slot; its timeRangeFilter default is unchanged and still depends only on initialFilters. The panel passes :show-time-range-filter="false" and sets updatedAfter to a 30-days-ago cutoff in its own query variables, both for the first query and on every search-variables-updated event, ignoring whatever updatedAfter the filtered search emits. The cutoff comes from a new getSessionRetentionCutoff() helper in duo_agents_platform/utils, which the filtered search's own getUpdatedAfter() now uses too, so the two compute the same date. The index page never sets the prop or initialFilters.updatedAfter, so its dropdown still defaults to "All".

This is behind the duo_panel_session_inbox feature flag, so no changelog entry is added.

References

• Parent feature: #616496

Screenshots or screen recordings

Before	After

The full-page Automate > Sessions index is unchanged (dropdown still present):

How to set up and validate locally

1. Enable the feature flag:

   Feature.enable(:duo_panel_session_inbox)

2. Open the Duo panel and its session inbox: the "All" / "Available" time-range dropdown no longer appears above the tabs, and a session last updated more than 30 days ago is not listed (the network tab shows updatedAfter set on the inbox query).
3. Open Automate > Sessions (the full-page index): the dropdown still renders exactly as before, defaulting to "All", and the >30-day session appears there.

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