Add seat assignment model validation

What does this MR do and why?

Add seat assignment model validation

Validating the presence of namespace_id so that we can eventually drop the DB not null constraint, to match business requirements

1. Add model validation 👈 we are here
2. Remove DB uniq index and replace - !185599 (merged)
3. Remove DB not null constraint - !185599 (merged)

• Docs: https://docs.gitlab.com/development/database/not_null_constraints/#dropping-a-not-null-constraint-on-a-column-in-an-existing-table
• Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/522974
• Spike: https://gitlab.com/gitlab-org/gitlab/-/issues/522973#note_2392698973

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

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