DMS Greeter Default Session Selector

### Feature Description

This will allow user to select which session to use if the user have multiple session regardless of last session used.

i.e User has 3 sessions installed Plasma, Niri, Hyperland.
Then User Can select which one should be priories.

### Use Case

Easier Way for user to access same session on devices which are used by multiple users.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

This will allow user to select which session to use if the user have multiple session regardless of last session used.

i.e User has 3 sessions installed Plasma, Niri, Hyperland.
Then User Can select which one should be priories.

### Alternatives/Existing Solutions

_No response_

The issue names no files, tests, or entry points. Start by locating the DMS Greeter flow that discovers installed compositor sessions and handles the last-used session. Done means a user with multiple installed sessions can choose which session to prioritize, regardless of the previously used session.

Written by the indexing model from the issue text.