Fix search term case sensitivity for transfer namespace

What does this MR do and why?

This MR fixes the case insensitive search for the project/group transfer namespace dropdown (see comment). It also improves the unit test coverage around this component's behavior.

It also addresses the following backstage polish comments:

• Remove unreferenced computed selectedNamespaceId (see comment).
• Fix invalid = [] since the variable is expected to actually be a string (see comment).

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Sign in
2. Visit a group you own and go to Settings > General
3. Go to the Transfer Group section under Advanced
4. Open the "Select parent group" dropdown
5. Enter part of a group's name you see using capital letters
6. The group should be included

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.