🧪 Add test for SurveyEngine empty node handling

[alfieprojectsdev]

• What: Added a unit test to field-logic/src/lib/engine.test.ts to verify that SurveyEngine.start() throws a "Survey has no nodes" error when the survey definition contains an empty list of nodes.
• Coverage: This test case covers the edge condition where a survey definition is technically valid (passes schema validation potentially) but has no nodes to execute, preventing a runtime error during start().
• Result: The test coverage for SurveyEngine.start() is now more robust, ensuring that empty surveys are handled gracefully with a clear error message.

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PR created automatically by Jules for task 13337167639846561904 started by @alfieprojectsdev

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