Block Multiple Agency Creation in CKB

[PaulaMerle]

Implement validation to prevent creation of multiple agencies within a single CKB deployment instance. CKB is designed to operate with one agency per deployment. Hide the agency creation button in the UI when an agency already exists to prevent users from attempting multiple agency creation.

This change will affect existing agency creation flows and workflows that manage agency state.

Acceptance criteria

• Add API validation to check if an agency already exists
• Hide agency creation button in the UI when an agency is present
• Return appropriate error message if creation is attempted when agency exists
• Document the constraint in API documentation

[PaulaMerle]

We might need to allow multiple Agency creation in case of Multi-Domain BYK usage because then multiple Agencies' data is collected into one CKB. It would be good to separate the data by the domains/Agencies.