refactor: centralize task lifecycle validation

[ash1shkumar]

📝 Description

This PR improves task workflow reliability by introducing reusable lifecycle validation helpers and consolidating task transition validation into a centralized validation layer.

Previously, task lifecycle validation checks were performed directly within workflow validation logic. As additional task states, workflow rules, and transition capabilities are introduced, maintaining consistent validation behavior can become increasingly difficult.

Changes Implemented

• Introduced reusable task status validation helper
• Added centralized task position validation helper
• Consolidated lifecycle validation logic into dedicated functions
• Standardized workflow validation checks
• Reduced duplication within task validation paths
• Improved separation between validation rules and request handling
• Enhanced maintainability of workflow validation logic

Benefits

• Improved task lifecycle consistency
• Reduced validation duplication
• Cleaner validation architecture
• Better maintainability
• Improved extensibility for future workflow states
• Easier implementation of future transition rules
• More predictable task validation behavior

🎯 Type of Change

• Refactoring
• Code Quality Improvement
• Maintainability Enhancement

✅ Testing

• Verified valid task statuses continue to pass validation
• Verified invalid task statuses are correctly rejected
• Verified valid task positions continue to pass validation
• Verified invalid task positions are correctly rejected
• Verified task title validation remains unchanged
• Verified task description validation remains unchanged
• Verified sanitization behavior remains intact
• Verified message validation functionality remains unaffected
• Verified feed item validation functionality remains unaffected
• Verified workflow behavior remains unchanged after refactoring

Fixes #208

Expected Labels

level3 NSoC'26

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