Clearly show "Message grouping" as disabled in edit state

[acco]

Context

When a user edits an existing sink, the Message Grouping card shows in a read-only state. We should make it even clearer that Message grouping is not editable, and that you must create a new sink to modify this behavior.

Problem

• There is no visual cue or explanatory text telling the user why the field can’t be changed.

Proposed solution

1. Chevron remains visible (to keep the layout consistent).
2. Hovering on the Message Grouping card title bar:

   • A tooltip appears:

     “Message grouping can’t be changed for an active sink.”

3. The dropdown cannot be opened.

[acco]

@davoclavo Good one to build on the work you've done so far!

[davoclavo]

Sounds good, I'll tackle this!