chore: estimates and issue parent id fixes

[NarayanBavisetti]

This PR resolves the following issues:

Backend:

• Implemented a validation for estimate points, limiting the value to a maximum of 20 characters.
• Enforced that the sequence ID accepts only whole numbers, excluding decimals (in the search endpoint).
• Included a validation to verify the issue's parent ID.

Frontend:

• Added validation while creating/updating estimates, if estimate point characters are more than 20 characters, then toast will be shown with this message: Estimate point cannot have more than 20 characters.