fix: improve input validation and retry flow in Productivity Pet

[jyotish6699]

Description

This PR improves the input validation and retry flow in the Productivity Pet CLI project.

Changes Made

• Added retry loops for invalid numeric inputs instead of restarting the entire interaction flow
• Rejected empty and whitespace-only task names
• Improved interaction continuity for task count and completed task inputs
• Preserved the original beginner-friendly procedural structure and emoji-based CLI style

Problem Fixed

Previously:

• Invalid inputs restarted the whole interaction
• Whitespace-only task names were accepted
• Users had to repeatedly restart sections after mistakes

Now:

• Users can retry immediately until valid input is entered
• Empty task names are properly rejected
• CLI interaction flow is smoother and more user-friendly

Closes #223

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[github-actions]

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[github-actions]

Thanks @jyotish6699! I've added you to the contributors list.