Add API tests, fix bugs, and add task assignment

[vansh01603]

Summary

Implemented the requirements for the take-home assignment.

Changes made

• Added comprehensive Jest + Supertest API tests
• Added endpoint coverage for task creation, listing, filtering, pagination, stats, update, delete, and completion
• Added tests for edge cases and validation
• Fixed the task completion service bug
• Added PATCH /tasks/:id/assign
• Added assignee validation
• Added tests for assignment, reassignment, missing assignee, empty assignee, and invalid assignee types
• Added BUG_REPORT.md
• Added SUBMISSION_NOTES.md

Test Results

• 25 tests passed
• 1 test suite passed
• 93.58% statement coverage
• 82.02% branch coverage
• 93.33% function coverage
• 93% line coverage

Additional Notes

The API uses the existing in-memory storage as provided in the assignment. No database changes were made.