Keep missing stored breakpoint groups selectable when editing responsive video styles

Problem/Motivation

A responsive video style stores the selected breakpoint group in configuration. If that breakpoint group is later removed, renamed, or no longer available, the edit form currently rebuilds the breakpoint group select from the active breakpoint manager only.

That means the stored value disappears from the available options even though it is still part of the saved configuration. This makes the form confusing for site builders and increases the risk of accidentally overwriting the existing configuration when saving the entity.

The edit form should remain resilient when referenced breakpoint groups are missing.

Steps to reproduce

1. Install responsive_video_style.
2. Create a responsive video style and save it with a valid breakpoint group.
3. Remove or rename that breakpoint group so it is no longer available from the breakpoint manager.
4. Edit the existing responsive video style.
5. Observe that the saved breakpoint group is no longer present in the select options.

Proposed resolution

When building the breakpoint group select options, preserve the stored breakpoint group if it is not present in the currently available breakpoint groups.

The missing stored option should remain selectable and should be labeled clearly, for example:

my_breakpoint_group (missing breakpoint group)

This keeps the current configuration visible, avoids accidental data loss on save, and gives site builders a clearer signal that the referenced breakpoint group must be restored or replaced.

Remaining tasks

• Update the responsive video style form to build breakpoint group options through a helper method.
• If the stored breakpoint group is missing, inject it into the select options with a clear label.
• Verify that existing responsive video styles with missing breakpoint groups can still be edited safely.

User interface changes

Yes.

If a stored breakpoint group is missing, the edit form will still show it in the select list with a label indicating that the breakpoint group is missing.

API changes

No.

This issue changes form behavior only.

Data model changes

No.

The saved configuration shape remains unchanged.