Creating a section before an existing section results in incorrect lock data for the new and existing section

Problem/Motivation

When adding a section before an existing section, there are several issues:

• the new section form shows the lock options, while we expect not to (see #3129009: Cannot add sections to default layout because of OutOfBoundsException: Invalid delta error)
• the lock settings are stored on the existing section

Steps to reproduce

I created a test to make this more visible.
To visually see the issues, comment out the assertions and check the browsertest output.

Proposed resolution

This issue exists because the check in layout_builder_lock_form_layout_builder_configure_section_alter() is not watertight.
There will always be lock configuration in this case, as the $delta refers to an already existing section.

So we need to know if it's a new or existing section.

Unfortunately there is currently no way to know if a section is being created or updated.
the property on the ConfigureSectionForm is protected and no public method exist to expose it.

So IMO the current "solution" (more a hacky way) is to do a similar check as layout builder does: check if a plugin_id is passed in the ConfigureSectionForm::buildForm() method to determine if we are adding or editing a section.
A better solution would obviously be to check this by a public method on the ConfigureSectionForm.

The patch contains this hacky fix and a rework of the existing multiple sections test to include the tests for this case.
It it makes more sense to add an extra test, that's also fine :)

Remaining tasks

Create a core issue to somehow expose the state of the section form.

User interface changes

none

API changes

none

Data model changes

none