fix(api): Improve block range validation in POI endpoint

[VolodymyrBg]

Description

Improves block range validation in the POI endpoint by implementing proper class-validator decorators. The change replaces manual validation with a custom IsGreaterThan decorator that ensures endBlock is always greater than startBlock when provided. This makes the validation more robust and consistent with the rest of the codebase's validation patterns.

Fixes #[issue_number] (if there is a related issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Changes made

• Added custom IsGreaterThan validation decorator
• Removed manual validation in the controller
• Fixed TODO comment regarding class validator functionality
• Improved type safety for block range validation

Checklist

• I have tested locally
• I have performed a self review of my changes
• Updated any relevant documentation
• Linked to any relevant issues
• I have added tests relevant to my changes
• Any dependent changes have been merged and published in downstream modules
• My code is up to date with the base branch
• I have updated relevant changelogs