fix: improve scenario dialogue closing and intent handling

[HansonL622]

Summary

• prevent duplicate closing responses by letting state advancement drive each scenario response exactly once
• treat changed requests and declined optional topics as valid conversation progress
• constrain scene endings to one short, natural in-role farewell without recap or learner evaluation
• add focused frontend and backend regression coverage

Verification

• npm run check:realtime-events
• npm run build
• backend targeted scenario dialogue tests: 15/15 passed

[fennoai]

No issues found in this review. The changed scenario prompt/state-machine behavior and realtime response handling were checked across code quality, performance, security, and documentation accuracy passes. Targeted verification passed: npm run check:realtime-events and ./mvnw -q -Dtest=ScenarioDialogueEventExtractorTest,ScenarioDialogueStateMachineTest,CustomSceneGeneratorTest test.

[yydounai1234]

LGTM