UI: asset schedule count and layout depend on which assets the caller may read

[henry3260]

Description

Since #72864 and #72862, GET /ui/next_run_assets/{dag_id} returns only the
scheduling assets the caller may read, while asset_expression keeps a hidden
slot for each unreadable one. AssetSchedule.tsx still derives both the total
and the layout from the returned events, so the UI now changes shape with the
caller's permissions.

For a Dag scheduled on assets A, B and C, a caller who may read only A sees:

• "1 of 1" instead of "1 of 3" — scheduledTotal is nextRunEvents.length
  (AssetSchedule.tsx:124).
• The single-asset layout instead of the popover, because of the
  nextRunEvents.length === 1 branch (AssetSchedule.tsx:207).
• No asset schedule at all if none of the three are readable, because the
  !nextRunEvents.length branch falls back to timetableSummary
  (AssetSchedule.tsx:128).

Use case/motivation

No response

Related issues

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Are you willing to submit a PR?

• Yes I am willing to submit a PR!

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