Specific error content when a department is blocked from signing up

## Context

#6739

We could add something re: departments we cannot serve in any context, like parl.gc.ca for example, to give potential users some feedback as to why they're not able to sign up for GC Forms. Often this is based on authorities we have (check with policy for content update).

This is based on entities we're blocking from registering here: https://github.com/cds-snc/platform-forms-client/blob/main/lib/tests/validation/validation.test.ts

Start with lib/tests/validation/validation.test.ts and review the entities currently blocked from registering. Trace where signup validation produces its response, then confirm the policy-approved wording for departments such as parl.gc.ca. Done means blocked departments receive specific feedback explaining why signup is unavailable, with validation tests covering the behavior.

Written by the indexing model from the issue text.