Allow subcontext scope to override parent scope
## Problem/motivation
Follow-up to:
#3576092
## Solution
From the other issue:
Discussion with Emma+Aidan at UX sync:
- Toggle (true/false) is defaulted to "Same as parent" (true) right above Global and then hide all of them
- Description: "Disable to customize scope."
Here's a [plan from cursor](https://www.drupal.org/files/issues/2026-04-21/subcontext_scope_inheritance_51216439.plan_.md) that can be updated to reflect ^
## Tasks
- [x] Add code
- [x] Review code
- [ ] Manually test code
## AI usage
- [ ] AI assisted issue
- [ ] AI assisted code
- [x] AI generated code
issue