Update toast notification to reflect hiding a catalog item rather than deleting

What does this MR do and why?

With this MR, the wording for "soft deleting" a catalog item changes to "Hide from the AI Catalog", however the toast notification that appears will still read "Agent/Flow deleted", which may be confusing for users.

This MR

• Changes the toast notification to read "Agent/Flow hidden" when an item is soft deleted
• Updates the relevant tests accordingly

Screenshots or screen recordings

How to set up and validate locally

1. Create an agent/flow as a user with permission to hard delete catalog items (ie. as root in GDK)
2. Delete that agent/flow and select the "soft delete" or "hide" option

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #582876