fix(designer): duplicate unsaved survey questions

[mohiuddinshahrukh]

What changed

• Save the duplicated form view model instead of re-saving the original when using the duplicate action.
• Initialize question response option validity with an empty map so newly created, unsaved questions can be copied before they have been loaded from the domain model.
• Add focused coverage for the duplicate action and copying newly created question data.

Why

Part of #737. Duplicating a newly added survey question could fail before the questionnaire itself was saved. Saved questions worked because their response option validity was initialized during domain-model loading; newly created questions did not go through that path.

Testing

• flutter test test\features\forms\form_view_model_collection_actions_test.dart
• Manual Designer frontend test: added a new unsaved survey question, duplicated it before saving the parent questionnaire, and confirmed the duplicate appears correctly.

[github-actions]

Visit the preview URL for this PR (updated for commit 4e1e391):

• https://studyu-dev-app--pr829-fix-737-duplicate-un-nzicmp9l.web.app
• https://studyu-dev-designer--pr829-fix-737-duplicate-un-6fxaorfc.web.app

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