Deleting or renaming “Edit Subgraph Representation” should not be presented as an option

Steps to reproduce:
1. Create new Unity project
2. Open Package Manager window
3. Install Behaviour package (com.unity.behavior)
4. In Project window create Behaviour Graph (Create > Behaviour > Behaviour Graph)
5. Open Behaviour Graph
6. In Behaviour Graph > Inspector, press on “Edit Subgraph Representation” button
7. In empty field type “testing”
8. Change Variable from “Regular Text” to “GameObject”
9. Press “Confirm”
10. Back in Blackboard right-click on “Testing” GO Variable and select “Delete” from context menu
11. Observe the Variable

Actual results: The variable gets recreated and it is not clear to the user why
Expected results: The option to delete and rename subgraph variables are grayed out and there is a clear distinction between a subgraph variable and other variables.

Reproducible with versions: 1.0.13 (6000.0.62f1, 6000.2.10f1, 6000.3.0b9, 6000.4.0a4)

Tested on (OS): Ubuntu 22.04, macOS Silicon, Tahoe 26.0 (M1)

Notes:

• the same applies to renaming