Admin Portal: Clearer 'Search tenants' Prompt in Upload Center Tenant Picker

A small Admin portal copy fix reached QA: when creating or editing an Upload Center, the tenant picker's search box now reads "Search tenants..." instead of the generic "Search entities...", making the action clearer to admins.

The shared tenant-selection component used a hard-coded, generic search placeholder ("Search entities..."). In the Upload Center form, where admins specifically search for a tenant, this wording was inconsistent with the rest of the dialog and could be mildly confusing.

Admins configuring an Upload Center now see a search prompt that matches what they are actually doing (searching tenants). This is a cosmetic, text-only improvement with no change to behavior, data, or available options. No client-facing/end-user surfaces are affected.

• Added an optional searchPlaceholder prop to the shared TenantSelect component (apps/admin/src/components/forms/tenant-select.tsx), defaulting to "Search entities..." so existing usages are unchanged.
• Replaced the hard-coded search input placeholder with the new prop.
• Passed searchPlaceholder="Search tenants..." from the Upload Center form dialog (apps/admin/src/components/admin/upload-centers/UploadCenterFormDialog/index.tsx).
• Net change is 6 additions / 1 deletion across 2 files; carried into QA via the dev→qa merge (commit 20b5458).

Very low. The change is limited to a display string with a backward-compatible default, so other consumers of TenantSelect keep their existing placeholder. No logic, API, or data paths are touched. Mitigation: confirm the default placeholder still renders on other tenant pickers.

• Open the Admin portal Upload Center create/edit dialog and confirm the tenant picker's search box shows "Search tenants...".
• Verify typing in that box still filters tenants correctly and selection works.
• Spot-check another screen that uses the tenant select to confirm it still shows the default "Search entities..." placeholder and functions normally.