fix(ui): preserve polymorphic upload bulk select relation

[damnsamn]

What?

Fixes polymorphic upload fields with hasMany enabled when selecting existing documents via the list drawer checkbox flow.

Previously, bulk-selecting documents from a non-first upload collection saved the selected IDs with the first configured relationTo collection. For example, selecting documents from uploads-2 could save values as uploads-1 relationships with IDs that do not exist in uploads-1.

Why?

The single row-click selection path already receives the active row collection slug and saves the correct polymorphic relationship. The checkbox bulk-selection path only received selected IDs, so UploadInput fell back to its initial activeRelationTo, which is the first configured collection.

This caused broken polymorphic upload values whenever the drawer collection dropdown was changed before using checkbox selection.

How?

• Pass the list drawer's currently selected collection slug into onBulkSelect.
• Use that collection slug when building polymorphic upload values and loading selected docs.
• Add an e2e regression that creates an upload through the admin UI, switches the polymorphic upload drawer to the second collection, selects via checkbox, saves, and verifies the stored relationTo is preserved.

Fixes #16696