test: reach 100% coverage and fix navigation for parents without question

[devCrossNet]

What is accomplished by your PR?

Bug fix

getNavigation() threw a TypeError when a parent navigation item had no own question, but its children had. setActiveQuestionByNavigationItemId already supports this case. Now the parent is shown and hasValue / isValid are derived from its children. Parents without any question are still skipped.

Tests (19 → 28, coverage 100%)

• Move examples/misc.spec.ts to src/quaire.spec.ts (unit test, not an example) and add edge case tests: unknown question/navigation IDs, saving without active question, validation, navigation items without questions, restore without items
• Rename tests to describe the behaviour and fix copy-paste names (linear-flow-skip-question, extending-data-definition)
• Remove duplicated assertions in dependencies-between-questions
• Split the long custom-component-types test into three tests
• Enforce 100% coverage thresholds

Refactoring

• Extract _getQuestionFromItem to remove a dead null check in _validate

Is there something controversial in your PR?

• New protected method _getParentNavigationItemObject and a new parameter on _addNavigationItem. Subclasses that override _addNavigationItem must add the parameter.
• Not fixed: getNavigation() still crashes when a child comes before its parent in navigationItems.

Checklist

• Run unit tests to ensure all existing tests are still passing
• Add new passing unit tests to cover the code introduced by your PR
• Change documentation for the code introduced by your PR

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