fix: make forceResponsesEndpoint opt-in by default (#103)

**🐛 Fixes**
- Change the hidden `litellm-connector.forceResponsesEndpoint` setting to default to `false` so `/responses` routing only happens when users explicitly enable it.
- Update the config test to verify the new default and prevent accidental routing behavior regressions.

**📚 Documentation**
- Clarify in the JSDoc that `forceResponsesEndpoint` is opt-in, JSON-only, and not exposed in the Settings UI.
- Add a changelog entry for v2.1.5 to document the default correction.