Fix pb-message boilerplate templates ignored by generic container

[salman2013]

Summary

When inside a Problem Builder (or other StudioContainerWithNestedXBlocksMixin) container block, the "Add Component" buttons for pb-message variants (Message complete / incomplete / Max attempts) all create the same default block, ignoring the selected boilerplate template.
For detail look at this comment: #3058 (comment)

Root Cause

Two bugs in src/course-unit/add-component/AddComponent.tsx:

1. Rendering — the default case button click handler called handleCreateNewXBlock(type) without forwarding firstTemplate?.boilerplateName.
2. Handler — the default branch of handleCreateNewXBlock called handleCreateNewCourseXBlock(...) without boilerplate: moduleName, so the backend always used the default template.

Fix

• Pass firstTemplate?.boilerplateName from the button onClick to handleCreateNewXBlock.
• Forward it as boilerplate in the handleCreateNewCourseXBlock call inside the default handler branch.

Related

• Parent DEPR ticket: [DEPR]: Legacy Unit Editor openedx-platform#35261