Add session CRUD and duration tests

[BhoomikaC15]

Summary

Add tests for session endpoints and duration calculation.

Scope

• Create session with session_start and session_end
• Verify duration_min is computed correctly
• Verify legacy date + duration_min input still works
• Test update and delete session routes
• Test filtering by user_id and category_id

Acceptance Criteria

• Session creation works with ISO datetime input
• Duration is calculated from timestamps
• Existing legacy session format still behaves correctly