fix: add validations on grant input

[GAlexIHU]

Overview

Fixes #(issue)

Notes for reviewer

Summary by CodeRabbit

• Bug Fixes
  • Creating credit grants now validates input: amount must be greater than zero and an effective-at timestamp is required.
  • Invalid requests receive clear, field-specific validation messages, preventing grant creation from proceeding with incorrect data.
  • Users get earlier feedback on submission issues, improving reliability and reducing failed transactions.