Domains: Add uniqueness constraint and tests for DomainCategory model and CLI

[Samk13]

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Description

• Added unique=True constraint to the label field in DomainCategory model to enforce uniqueness at the database level.
• Modified domains_create CLI command to check for duplicates before creating a new domain category, providing feedback if the domain already exists.
• Added tests for DomainCategory model to validate creation, retrieval, and handling of duplicate labels.
• Enhanced test_cli_createdomain
• closes Duplicate Domain Creation Allowed Without Validation invenio-app-rdm#2796

Checklist

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