assignment submission

[sanjaysahu22]

• Fixed status filtering to use exact matching instead of substring matching.
• Corrected pagination offset calculation and added validation for page and limit.
• Fixed completeTask so it no longer resets task priority and made completion idempotent.
• Normalized enum inputs (status, priority) to be case-insensitive.
• Refactored the code by introducing a dedicated taskController to separate routing from business logic.