FINERACT-2291: Migrate Group activate, close, assignStaff, unassignStaff endpoints to typed command framework

[nidhiii128]

Description

JIRA: https://issues.apache.org/jira/browse/FINERACT-2291

Migrates four Group state-change endpoints to the typed command framework:

• POST /groups/{id}?command=activate
• POST /groups/{id}?command=close
• POST /groups/{id}?command=assignStaff
• POST /groups/{id}?command=unassignStaff
• POST /groups/{id}/command/unassign_staff (legacy dedicated path)

These endpoints now flow through Command/CommandHandler/CommandDispatcher
with typed request/response POJOs instead of JsonCommand. Legacy methods
retained on the service interface for backward compatibility with other
internal callers.

Pattern follows the merged StaffCreateCommand reference implementation:

• Empty-body Command with @DaTa + @EqualsAndHashCode(callSuper = true)
• CommandHandler<REQ, RES> with @Retry + fallback method
• Lombok throughout, Jakarta Validation on requests
• Permission checks in service impl (validateHasPermissionTo)

Note: run-api-backward-compatibility CI check may flag the response
shape change from CommandProcessingResult to typed responses
(intentional, part of FINERACT-2169 migration scope).

Checklist

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