Add tests for insights endpoints

[BhoomikaC15]

Summary

Add tests for the analytics endpoints in insights.py.

Scope

• Test /insights/personal-baseline/<user_id>
• Test /insights/burnout/<user_id>
• Test /insights/recommendations/<user_id>
• Test behavior when a user has no sessions

Acceptance Criteria

• Endpoints return expected JSON shape
• Metrics are computed from stored sessions
• Empty-session cases return the expected response